The geometry classes define a hierarchy as follows:
Geometry (non-instantiable)
Point (instantiable)
Curve (non-instantiable)
LineString (instantiable)
Line
LinearRing
Surface (non-instantiable)
Polygon (instantiable)
GeometryCollection (instantiable)
MultiPoint (instantiable)
MultiCurve (non-instantiable)
MultiLineString (instantiable)
MultiSurface (non-instantiable)
MultiPolygon (instantiable)
Some of these classes are abstract (non-instantiable). That is, it is not possible to create an object of these classes. Other classes are instantiable and objects may be created of them. Each class has properties and instantiable classes may have assertions (rules that define valid class instances).
Geometry is the base class. It's an abstract class. The instantiable subclasses of Geometry are restricted to zero-, one-, and two-dimensional geometric objects that exist in two-dimensional coordinate space. All instantiable geometry classes are defined so that valid instances of a geometry class are topologically closed (that is, all defined geometries include their boundary).
The base Geometry class has subclasses for Point, Curve, Surface and GeometryCollection:
Point represents zero-dimensional objects.
Curve represents one-dimensional objects, and has subclass LineString, with sub-subclasses Line and LinearRing.
Surface is designed for two-dimensional objects and has subclass Polygon.
GeometryCollection has specialized zero-, one-, and two-dimensional collection classes named MultiPoint, MultiLineString, and MultiPolygon for modelling geometries corresponding to collections of Points, LineStrings, and Polygons, respectively. MultiCurve and MultiSurface are introduced as abstract superclasses that generalize the collection interfaces to handle Curves and Surfaces.
Geometry, Curve, Surface, MultiCurve, and MultiSurface are defined as non-instantiable classes. They define a common set of methods for their subclasses and are included for the reason of extensibility.
Point, LineString, Polygon, GeometryCollection, MultiPoint, MultiLineString, and MultiPolygon are instantiable classes.