The video footage can be combined with new commentary and new footage, but most of the footage of a compilation film consists of archive or stock footage that has been used in earlier, different movies. Sometimes it can also be older material shot again, but with a higher budget.
The quality of these types of films is variable. Sometimes the archive footage is just edited behind each other, without adding anything new. Because of running time limits some footage can be shortened or expanded with short, new footage in an effort to make everything seamlessly flow together into each other. These transitions are not always unnoticeable. (source : wikipedia)