Examples are lines, curves, meshes and boundary representations.
Classes
Class  Description  

AngularDimension 
Represents a dimension of an entity that can be measured with an angle.
This entity refers to the geometric element that is independent from the document.  
AnnotationBase 
Provides a common base class to all annotation geometry.
This class refers to the geometric element that is independent from the document.  
ArcCurve 
Represent arcs and circles.
ArcCurve.IsCircle returns true if the curve is a complete circle.  
AreaMassProperties 
Contains static initialization methods and allows access to the computed
metrics of area, area centroid and area moments in closed
planar curves, in meshes, in surfaces, in hatches and in boundary representations.
 
BezierCurve 
Represents a Bezier curve.
Note: as an exception, the bezier curve is not derived from Curve.  
Brep 
Boundary Representation. A surface or polysurface along with trim curve information.
 
BrepEdge 
Represents a single edge curve in a Brep object.
 
BrepFace 
Provides stronglytyped access to brep faces.
A Brep face is composed of one surface and trimming curves.  
BrepLoop 
Represent a single loop in a Brep object. A loop is composed
of a list of trim curves.
 
BrepRegion 
Represents a brep topological region that has sides.
 
BrepRegionFaceSide 
Represents a side of a BrepRegion entity.
 
BrepTrim 
Brep trim information is stored in BrepTrim classes. Brep.Trims is an
array of all the trims in the brep. A BrepTrim is derived from CurveProxy
so the trim can supply easy to use evaluation tools via the Curve virtual
member functions.
Note well that the domains and orientations of the curve m_C2[trim.m_c2i]
and the trim as a curve may not agree.
 
BrepVertex 
Brep vertex information
 
ClippingPlaneSurface 
Represents a planar surface that is used as clipping plane in viewports.
 
Curve 
Represents a base class that is common to most RhinoCommon curve types.
A curve represents an entity that can be all visited by providing a single parameter, usually called t.  
CurveProxy 
Provides stronglytyped access to Brep edges.
 
DetailView 
Represents a view of the model placed on a page layout.
 
Extrusion 
Represents an extrusion, or objects such as beams or linearly extruded elements,
that can be represented by profile curves and two miter planes at the extremes.
 
GeometryBase 
Provides a common base for most geometric classes. This class is abstract.
 
Hatch 
Represents a hatch in planar boundary loop or loops.
This is a 2d entity with a plane defining a local coordinate system.
The loops, patterns, angles, etc are all in this local coordinate system.
The Hatch object manages the plane and loop array
Fill definitions are in the HatchPattern or class derived from HatchPattern
Hatch has an index to get the pattern definition from the pattern table.
 
InstanceDefinitionGeometry 
Represents the geometry in a block definition.
 
InstanceReferenceGeometry 
Represents a reference to the geometry in a block definition.
 
Interpolator 
Exposes a set of standard numeric interpolation algorithms.
 
Leader 
Represents a leader, or an annotation entity with text that resembles an arrow pointing to something.
This class refers to the geometric element that is independent from the document.  
Light 
Represents a light that shines in the modeling space.
 
LinearDimension 
Represents a linear dimension.
This entity refers to the geometric element that is independent from the document.  
LineCurve 
Represents a linear curve.
 
Matrix  
Mesh 
Represents a geometry type that is defined by vertices and faces.
This is often called a facevertex mesh.  
MeshingParameters 
Represents settings used for creating a mesh representation of a brep or surface.
 
MeshPart 
Represents a portion of a mesh for partitioning
 
MeshPoint 
Represents a point that is found on a mesh.
 
MorphControl 
Represents a geometry that is able to control the morphing behaviour of some other geometry.
 
NurbsCurve 
Represents a Non Uniform Rational BSplines (NURBS) curve.
 
NurbsSurface 
Represents a Non Uniform Rational BSplines (NURBS) surface.
 
OrdinateDimension 
Represents the geometry of a dimension that displays a coordinate of a point.
This class refers to the geometric element that is independent from the document.  
Particle 
Represents a simple particle.
This base class only defines position and display properties (size, color, bitmap id). You will most likely create a class that derives from this particle class to perform some sort of physical simulation (movement over time or frames).  
ParticleSystem  
PlaneSurface 
Represents a plane surface, with plane and two intervals.
 
Point 
Represents a geometric point.
This is fundamentally a class that derives from GeometryBase and contains a single Point3d location.  
Point3dGrid 
Represents a rectangular grid of 3D points.
 
PointCloud 
Represents a collection of coordinates with optional normal vectors and colors.
 
PointCloudItem 
Represents a single item in a pointcloud. A PointCloud item
always has a location, but it has an optional normal vector and color.
 
PolyCurve 
Represents a curve that is the result of joining several (possibly different)
types of curves.
 
Polyline 
Represents an ordered set of points connected by linear segments.
Polylines are closed if start and end points coincide.  
PolylineCurve  
RadialDimension 
Represents a dimension of a circular entity that can be measured with radius or diameter.
This entity refers to the geometric element that is independent from the document.  
RevSurface 
Represents a surface of revolution.
Revolutions can be incomplete (they can form arcs).  
RTree 
Represents a spatial search structure based on implementations of the
Rtree algorithm by Toni Gutman.
 
RTreeEventArgs 
Represents event data that is passed when when an item that meets certain
criteria is found and the passed RTree event is raised.
 
SpaceMorph 
Represents a spacial, Euclidean morph.
 
SumSurface 
Represents a sum surface, or an extrusion of a curve along a curved path.
 
Surface 
Represents a base class that is common to most RhinoCommon surface types.
A surface represents an entity that can be all visited by providing two independent parameters, usually called (u, v), or sometimes (s, t).  
SurfaceCurvature 
Maintains computed information for surface curvature evaluation.
 
SurfaceProxy 
Provides a base class to brep faces and other surface proxies.
 
SweepOneRail 
Utility class for generating breps by sweeping cross section curves over
a single rail curve.
 
SweepTwoRail 
Utility class for generating breps by sweeping cross section curves over
two rail curves.
 
TextDot 
Represents a text dot, or an annotation entity with text that always faces the camera and always has the same size.
This class refers to the geometric element that is independent from the document.  
TextEntity 
Represents text geometry.
This class refers to the geometric element that is independent from the document.  
Unroller 
Represents the operation of unrolling a single surface.
 
VolumeMassProperties 
Contains static initialization methods and allows access to the computed
metrics of volume, volume centroid and volume moments in
in solid meshes, in solid surfaces and in solid (closed) boundary representations.

Structures
Structure  Description  

Arc 
Represents the value of a plane, two angles and a radius in
a subcurve of a threedimensional circle.
The curve is parameterized by an angle expressed in radians. For an IsValid arc the total subtended angle AngleRadians() = Domain()(1)  Domain()(0) must satisfy 0 < AngleRadians() < 2*Pi The parameterization of the Arc is inherited from the Circle it is derived from. In particular t > center + cos(t)*radius*xaxis + sin(t)*radius*yaxis where xaxis and yaxis, (part of Circle.Plane) form an othonormal frame of the plane containing the circle.  
BoundingBox 
Represents the value of two points in a bounding box
defined by the two extreme corner points.
This box is therefore aligned to the world X, Y and Z axes.  
Box 
Represents the value of a plane and three intervals in
an orthogonal, oriented box that is not necessarily parallel to the world Y, X, Z axes.
 
Circle 
Represents a circle in 3D.
The values used are a radius and an orthonormal frame of the plane containing the circle, with origin at the center. The circle is parameterized by radians from 0 to 2 Pi given by t > center + cos(t)*radius*xaxis + sin(t)*radius*yaxis where center, xaxis and yaxis define the orthonormal frame of the circle plane.  
ComponentIndex 
Represents an index of an element contained in another object.
 
Cone 
Represents the center plane, radius and height values in a right circular cone.
 
ControlPoint 
Represents controlpoint geometry with threedimensional position and weight.
 
Cylinder 
Represents the values of a plane, a radius and two heights on top and beneath
that define a right circular cylinder.
 
Ellipse 
Represents the values of a plane and the two semiaxes radii in an ellipse.
 
Interval 
Represents an interval in onedimensional space,
that is defined as two extrema or bounds.
 
Line 
Represents the value of start and end points in a single line segment.
 
MeshFace 
Represents the values of the four indices of a mesh face quad.
If the third and fourth values are the same, this face represents a triangle.  
Plane 
Represents the value of a center point and two axes in a plane in three dimensions.
 
Point2d 
Represents the two coordinates of a point in twodimensional space,
using Doubleprecision floating point numbers.
 
Point2f 
Represents the two coordinates of a point in twodimensional space,
using Singleprecision floating point numbers.
 
Point3d 
Represents the three coordinates of a point in threedimensional space,
using Doubleprecision floating point values.
 
Point3f 
Represents the three coordinates of a point in threedimensional space,
using Singleprecision floating point numbers.
 
Point4d 
Represents the four coordinates of a point in fourdimensional space.
The W (fourth) dimension is often considered the weight of the point as seen in 3D space.  
Quaternion 
Represents the four coefficient values in a quaternion.
The first value a is the real part, while the rest multipies i, j and k, that are imaginary. quaternion = a + bi + cj + dk  
Ray3d 
Represents an immutable ray in three dimensions, using position and direction.
 
Rectangle3d 
Represents the values of a plane and two intervals
that form an oriented rectangle in three dimensions.
 
Sphere 
Represents the plane and radius values of a sphere.
 
Torus 
Represents the value of a plane and two radii in a torus that is oriented in threedimensional space.
 
Transform 
Represents the values in a 4x4 transform matrix.
This is parallel to C++ ON_Xform.  
Vector2d 
Represents the two components of a vector in twodimensional space,
using Doubleprecision floating point numbers.
 
Vector2f 
Represents the two components of a vector in twodimensional space,
using Singleprecision floating point numbers.
 
Vector3d 
Represents the three components of a vector in threedimensional space,
using Doubleprecision floating point numbers.
 
Vector3f 
Represents the three components of a vector in threedimensional space,
using Singleprecision floating point numbers.

Enumerations
Enumeration  Description  

BlendContinuity 
Used in curve and surface blending functions
 
BrepLoopType 
Each brep loop has a defined type, e.g. outer, inner or point on surface.
 
BrepSolidOrientation 
Enumerates all possible Solid Orientations for a Brep.
 
BrepTrimType 
Each brep trim has a defined type.
 
ComponentIndexType 
Defines enumerated values to represent component index types.
 
Continuity 
Provides enumerated values for continuity along geometry,
such as continuous first derivative or continuous unit tangent and curvature.
 
CurveEnd 
Defines the extremes of a curve through a flagged enumeration.
 
CurveEvaluationSide 
Defines enumerated values for the options that defines a curve evaluation side when evaluating kinks.
 
CurveExtensionStyle 
Defines enumerated values for styles to use during curve extension, such as "Line", "Arc" or "Smooth".
 
CurveKnotStyle 
Defines enumerated values for knot spacing styles in interpolated curves.
 
CurveOffsetCornerStyle 
Defines enumerated values for all implemented corner styles in curve offsets.
 
CurveOrientation 
Defines enumerated values for closed curve orientations.
 
CurveSimplifyOptions 
Enumerates the options to use when simplifying a curve.
 
EdgeAdjacency 
Enumerates all possible Topological Edge adjacency types.
 
ExtrudeCornerType 
Corner types used for creating a tapered extrusion
 
IsoStatus 
Defines enumerated values for isoparametric curve direction on a surface, such as X or Y,
and curve sides, such as North or West boundary.
Note: odd values are all xconstant; even values > 0 are all yconstant.  
LightStyle 
Defines enumerated values to represent light styles or types, such as directional or spotlight.
 
LoftType 
Specifies enumerated constants for all supported loft types.
 
MeshingParameterStyle 
Type of Mesh Parameters used by the RhinoDoc for meshing objects
 
MeshType 
Defines enumerated values for various mesh types.
 
PipeCapMode  constansts used for CreatePipe functions  
PlaneFitResult 
Enumerates all possible outcomes of a LeastSquares plane fitting operation.
 
PointContainment 
Defines enumerated values for closed curve/point spatial relationships.
 
PointFaceRelation 
Enumerates the possible point/BrepFace spatial relationships.
 
RegionContainment 
Defines enumerated values for closed curve/closed curve relationships.
 
TextJustification 
Specifies enumerated constants used to indicate the internal alignment and justification of text.
 
TransformSimilarityType 
Lists all possible outcomes for transform similarity.
