In terms of animation or image production, a composite is the result of blending together disparate images, motions, elements, effects, and so on, into a final representative image or shot. A composite may consist of separate passes for lights, depth, color, reflection, refraction, and so on, for each element in a scene. As you can imagine, a composite has the potential to get very complicated, and many times, that often occurs. However, just as often, a composite can be as simple as laying one element on top of a background for a finished image or shot. It really depends upon the requirements of the creative brief.
As a Cinema 4D user, you may be familiar with the ability to share project and render information with Adobe After Effects, which is a compositing application used quite extensively in television and film. Quite often its best to use After Effects for the heavy lifting of manipulating elements for the final composite, and use Cinema 4D for the generation of initial elements. Its typically faster to manipulate something in a 2D application than it is a 3D application, though that is not always the case.