Composite Operators, or COPs in Houdini allow you to composite elements much like any other compositor. The only difference is that COPs within Houdini is network-based. Like anything else in Houdini, you can generate pixel data and then pipe that through a series of filter nodes to modify the output.
This tutorial walks you through using the composite operators to generate normal maps right inside Houdini. The benefit is that COPs are lightweight and easy to edit. This means that you are able to make a lot of versions using a combination of composite operators and SOPs (Surface Operators, for geometry) to get the images you need.
It is really like layering images together in a compositor, only you are doing it in 3D right inside of Houdini.
