"MotS" is short for "Mysteries of the Sith," the addon missions disc for Jedi Knight.
See
www.lucasarts.com) (
http://www.lucasarts.com) for more info on this product. There is also a demo you can download. Unlike many addon packs, MOTS is more like it's own game, but using a modified version of JK's (Sith) engine and continuing the storyline (as if Kyle had gone the Light Side route at the end of JK).
In my opinion, MOTS is superior to JK at least in the MP department which is much more complex and varied (for example, many more MP maps are included, plus there are player classes and more weapons and force powers).
The game seems to have some more bugs than JK did (not many, but still some) that are annoying, and the game does not have support for high res sounds. The cutscenes are of lower quality as well.
However, the game has better graphics (support for colored lighting) as well as some neat features like audio taunts and more weapons and enemies.
The SP portion is shorter and I think less polished than JK's, but it's MP portion is much better.
I highly recommend you try it out, and see for yourself.
Kurgan