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.
See the 
Copy to Points shelf tool and the Copy node help for more information on copying.
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.
Subtopics
Getting started
| Simple duplication | ||
| Copying geometry to points | ||
| Instancing |
Next steps
| Copy stamping | ||
| Copy stamping tutorial | ||
| Loading instance geometry from disk | ||
| Varying copies/instances |
Guru level
| Copying and instancing point attributes |