Garindan and Pedro have alot more posts than I do.
Jk is a First Person Shooter. The main character is Kyle Katarn For weapons you have a lightsaber, fists, Bryar pistol, stormtrooper rifle, Thermal detonator, bowcater, repeater, rail detonator, timed mines, the concussion rifle and a wide range of force powers. "light and dark"
The levels are large and numerous and have a very wide range of characters. You also fight 7 dark jedi in it.
MotS is an expansion for jedi Knight. It has many improvements in gameplay but it's kind of a "back ally story". The best part of these two games, though, is that they are editable and have many mods and fan made levels for them. You could probably get them off ebay for 15 bucks all together. Or if you pay for purchase price+shipping, I'll have a copy sent to you.