There are currently four ways in Blender to procedurally duplicate objects. These options are located in thepanel.
This creates an instance of all children of this object on each vertex (for mesh objects only).
This creates an instance of all children of this object on each face (for mesh objects only).
This creates an instance of the group with the transformation of the object. Group duplicators can be animated using actions, or can get a Proxy.
For animated objects, this creates an instance on every frame. As you will see on this topic’s subpage, this is also a very powerful technique for arranging objects and for modeling them.