The Jedi guardians are those Jedi who aren't afraid to take out their lightsaber when the situation demands it. They shouldn't be thought of as warriors, since "wars not make one great," after all. Indeed, the life of a Jedi guardian should be dedicated to peace, the minimization of bloodshed, and serene meditation on the meaning of the Jedi Code.
I'm kidding, of course. Guardians are meant to kill, kill, kill; that's what they're good for, and that's what you'll be able to do with relative ease if you choose to upgrade your character to this class. Guardians are not as strong in the Force as the other Jedi classes, but are much more powerful when it comes to wielding a lightsaber, and are more physically resilient as well. A guardian is roughly analogous to a soldier.
The Jedi guardian's class-specific feat is Force jump, which is a combat-oriented feat that allows a Jedi to cover massive amounts of ground and deliver a powerful blow when combat begins. The Jedi guardian is also the only class that is allowed to obtain lightsaber specialization, which gives each lightsaber attack +2 damage.
Vitality: +10 per Level
Force Points: +4 per level
Feat Progression: Levels 1, 3, 6, 7, 9, 12, 13, 15, 18
Skill points progression: (2 + Intelligence Modifier) / 2
The sentinel class is supposedly a balanced, diplomatic class, involved in settling conflicts between parties and arbitrating grievances through negotiations. Given the nature of KOTOR, though, most of a Jedi sentinel's negotiations will be of the aggressive variety, but not to worry; Sentinels are quite adept at handling themselves in combat.
The Jedi sentinel's unique feat is probably the most useful of the three: as a sentinel increases in level, he or she will become increasingly resistant to mind-affecting attacks, such as attacks that cause fear, or stun your character. This will become quite important when battling the Dark Jedi later on in the game, since they will quite often resort to these kinds of attacks when using the Force powers.
As a note, characters who advance through Taris as a soldier, and who want to proceed through the game as a combat-oriented character, may want to consider the sentinel as an alternative to the guardian class. While you do lose out on lightsaber specialization, which hurts, you'll gain more Force points, and will be able to increase your skills a bit faster than you were previously. And not to put too fine a point on it, but Force immunity is almost always going to be a better feat to have than Force jump, especially in the last few stages of the game.
Vitality: +8 per Level
Force Points: +6 per level
Feat Progression: Levels 1, 3, 6, 9, 12, 15, 18
Skill points progression: (4 + Intelligence Modifier) / 2