Geometry is the root class of the hierarchy.
A Feature is a Property List, some of whose properties are of type geometry. Specific classes of geographic feature are created by subtyping from the GML Feature class in the application namespace. A GeometryCollection is a geometry that is a collection of 1 or more geometries. All the elements in a GeometryCollection must be in the same Spatial Reference. This is also the Spatial Reference for the GeometryCollection.