Star Wars: The Old Republic offers up eight different classes, and each of these classes can split off into two different Advanced Classes. This design allows for a lot of flexibility and you'll soon realize you're able to specialize in either a damage or a support role no matter what Advanced Class you end up picking, well, almost. Enter the Jedi Sentinel, a dual-wielding Advanced Class available to the Jedi Knight. Like any Jedi Knight, the Sentinel is committed to protecting others -- but sometimes the best defense is a great offense, which is why the Sentinel comes equipped with two lightsabers and three specs focused entirely on dealing damage.
The Sentinel uses the Focus resource as its main mechanic. As the Sentinel attacks, he builds Focus, and certain abilities can be activated to spend that Focus. Additionally, Sentinels build stacks of Centering when using Focus spending abilities or defeating enemies in combat. You can gain up to 30 stacks of Centering and can spend all 30 stacks on a variety of short duration self or team-wide buffs.
Playing as Watchman:
Watchman is a great set to play in PvE. Early on, when your talent points generally don't make the most impact, Focused Slash from Watchman will give you the most bang for your buck no matter what specialization you ultimately end up going down. Obviously, if you plan on actually going down Watchman it just brings you that much closer to goodies like Overload Saber and Merciless Slash. The front-loaded damage is still pretty good early on in Watchman, but what is great is the extra mitigation provided to you from the bits of self-healing you can do with abilities like Zen in Juyo form or talents like Merciless Zeal. You'll want to open up with the standard Force Leap to Zealous Strike to get as much Focus at the beginning of the fight as possible. Keep Cauterize up (but let it run its course) on your target and keep Overload Saber going as often as possible once you pick it up at level 20. Of course, once Merciless Slash comes into the picture you'll want to keep that going as well. Don't neglect the talent Watchguard, as it will reduce the cooldown of Force Kick (your interrupt) to 6 seconds, affording you valuable additional survivability in many solo situations and allowing you to take some of the pressure off the tank in group situations.
Watchman is a great specialization to go down as a Sentinel, but it's really all about juggling cooldowns, both active ability cooldowns and optimizing your passive proc-based cooldowns. Also, be sure to stack Accuracy and Crit/Surge to take advantage of the critical heavy nature of this specialization. Accuracy (up to 110%) should be your first priority. If you can't hit, you can't crit!
Playing as Combat:
Combat is a bit more straightforward than Watchman, but that isn't saying much since Sentinels generally have complex skill rotations compared to most classes in the game. With that said, you're mainly concerned with feeding your hunger for Focus as you'll be running near empty a lot of the time with the way you tend to dump it. Unlike Watchman, you'll want to open up with Force Leap into Blade Rush so that you can benefit from the Ataru strike on Zealous Strike. From there, it's a matter of maximizing the use of the Precision Slash buff and Zen as often as possible. This means Precision Slash to Master Strike on cooldown and Precision Slash to Blade Storm/Blade Rush spam when Master Strike is down. Like Watchman, if you time your proc effects and active buff abilities correctly, you'll be putting out some significant damage as a Combat specialized Sentinel.
Playing as Focus:
Focus is truthfully more of a PvP specialization than a PvE specialization, but as far as PvE goes it's probably the most standard of the lot. You'll want to use Force Exhaustion and Stais on cooldown. Spam Slash along with Zen whenever you can when facing a single target or Sweep when facing a group of enemies. Ideally, you'll want to weave Sweeps in between applications of Force Stasis and Force Exhaustion to take advantage of the significant damage buff provided by the Singularity talent.
The Sentinel Wrap-Up
Sentinels are a user-intensive class that really brings the pain when wielded in the right hands. They aren't as straightforward as some of the game's other classes, but if you take the time to master this class you'll find it to be a very rewarding experience. However, the major disadvantage to playing a Sentinel is the lack of flexibility afforded to other Advanced Classes. Sure, you can pick from a number of different ways to kill things, but if you feel like you might want to alternate between tanking and doing damage, you should definitely consider the Jedi Guardian instead.