Cogs control all the functions in Jedi Knight - weapons, force powers, anything the player interacts with, or moves in-game, etc. Cog is a 'programming language' derived from C, created to make things happen in JK/MotS. There are tutorials in Massassi's Tutorial section that explain how to put door cogs in, camera cogs, etc. Pele wrote an
excellent beginner's guide to cogs a while ago, you can find that here:
http://www.jediknight.net/darkfall/dfsept98/cogframe.html
My (unfinished) tutorial is available here:
http://battle4naboo.hypermart.net/tut1.html
You will probably hear the term JKSpecs used a lot. There is a link to it on the navbar on Massassi's main page. It has a complete list of all cogs and their functions - some are not complete, but Millenium (
http://millennium.massassi.net ) has the rest.
I recommend reading Pele's guide thoroughly, and with JKSpecs in hand, learning Cog slowly. The only way to learn is by experience - it may seem hard, but take a look at LEC's cogs and figure out what they did, by referring to verb fuctions in JKSpecs.
-Jipe