Here's a basic definition:

Composite: The manipulated combination of at least two source

images to produce an integrated result.

By far, the most difficult part of this process is producing the integrated result - an image which doesn't betray that its creation is owed to multiple source elements.

In particular, we are usually attempting to produce (sequences of) images which could have been believably photographed without the use of any post-processing. Colloquially, it should look 'real'. Even if the elements in the scene are obviously not real, one must be able to believe that everything in the scene was photographed at the same time, by the same camera.

We will be discussing the manipulations needed to achieve this combination, and the various tools necessary to achieve the desired result. In the digital world, which is the world we're interested in for the bulk of today's discussion, these tools are specifically the software needed to create a composite. Keep in mind that compositing was being done long before computers entered the picture (pardon the pun). Optical compositing is still a valid and often-used process, and many of the techniques and skills developed by optical compositors are directly applicable to the digital realm ( in many cases, digital techniques owe their origin directly to optical methodologies ).

Finally, remember that every person who views an image has a little expert he carries around with him. This expert, the subconscious, has spent a lifetime learning what looks 'real'. Even if, consciously, the viewer is unable to say why a composite looks wrong, the subconscious can notice 'warning signs' of artificiality. Beware.