Are you looking for a Star Wars: The Old Republic Guide? Well, unfortunately I can't write a complete one here, to cover all the slices of game content. However, in this post I'm going to provide you with a quick overview on the SWTOR classes.
By reading this brief Star Wars: The Old Republic Guide, you will understand without reading too much what role can each SWTOR class fulfill, and hopefully you will make the right choice when creating your character and choosing a specialization tree.
If you choose to play on the good side of the force, joining the Galactic Republic faction, you can choose from four different main classes: Jedi Knight, Jedi Consular, Smuggler and Trooper.
The Jedi Knight. This class excels at close combat. As a Jedi Knight you can choose to be Guardian (tank), wielding one lightsaber and heavy armor or Sentinel (melee DPS) wielding two one-handed lightsabers and inflicting high amounts of close range DPS. As a JK, you will be relying specially on lightsaber attacks, and most of them have awesome animation.
The Jedi Consular. This type of jedi, is more of a caster type, speaking in MMO terms. You can follow two paths as a Jedi Consular, Shadow and Sage. If you're going to follow the Sage specialization, depending on the skill tree that you will choose, you can become either a force-healer of a force-wielding damage dealer. The Shadow Jedi Consulars on the other hand are double-blade lightsaber damage dealers, which also receive a stealth ability and stealth-based attacks.
The Smuggler. There's no doubt this is the cunning class of the Republic, with a solid arsenal of crowd control and crippling attacks. As a Smuggler you can be a Gunslinger, dual-wielding two blasters, focusing on precise ranged DPS, or you can choose the Scoundrel specialization, which can either act as a healer or off-healer, or as a DPS class with stealth and stealth-based ranged attacks.
The Trooper. The last class to be mentioned in this Star Wars: The Old Republic guide for the Republic faction is the heavy assault, ranged DPS class - the Trooper. Although they use heavy machine guns or rifles, Troopers can cover all the roles in the game, tank, healer or heavy DPS, depending on advancement specialization and skill tree they choose. The two paths a Trooper can choose from are Commando and Vanguard. As a Commando you can be either an expert gunner relying on a heavy assault cannon, or a combat medic. On the other hand, as a Vanguard you can become a medium ranged DPS, with quite a few interesting short range attacks, or a shield specialist, which boosts their armor factor becoming able to sustain plenty of heavy damage and hold aggro - that would be a tank role.
My choice: I like the Smuggler class a lot. Seems like a really flexible class and has quite a few cool moves that really stuck to me, and with proper training and equipment, it makes hell of a DPS.
Do you wish to master each side of SWTOR content, playing a Galactic Republic class?