Depends which jailbreak.
As NiLuJe pointed out, the Wiki has
lots of information about how the Kindle works and what the jailbreak does. Our "Start Here" master index (the sticky one right in this subforum) contains links to Wiki indexes where most things eventually get consolidated.
Failing that, you could open up the jailbreak and examine it -- you will notice that it is mostly bash scripts.
Basic overview:
It puts a MobileRead shared certificate for validating signed updates (you can create a corresponding update_*.bin using kindletool), it bundles in the MKK (MobileRead Kindlet Kit, formerly known as the Kindlet jailbreak) which contains a Test Kindle certificate for running non-Amazon kindlets, and it includes "gandalf", an entry point for quickly gaining a root shell.
But of course, that is useless trivia, in order to do anything practical, which you no doubt want, you will have to ask a more pointed question, or trawl the excellent Wiki.