This page has three simple demos of instancing. Instancing is a feature that lets you draw many meshes with one call, which is particularly useful for drawing billboards or other simple shapes.
Rotation animation with instancing
Billboard instancing
A variant of this is using instanced arrays, where a buffer provides information per instance:
Instanced arrays
Instancing bunny
Rotation-animation-with-instancing.tar.gz
Billboard-instancing.tar.gz
Billboard-instancing-with-instanced-arrays.tar.gz
Instancing-bunny.tar.gz