You can use copies (real geometry) and instances (geometry that is only loaded or created at render time) to achieve complex effects such as crowds, flocks, and other groups.
Copying vs. instancing
The copy node creates copies of the source geometry on each point of the destination geometry. It creates actual geometry in the scene you can then model with, but it increases the amount of processing Houdini has to do to cook and display the scene.
Point instancing copies the geometry at render time, so each copy only exists momentarily as its area of the image is rendered. The geometry only appears in the render, not in the viewer.