There are 3 starting classes, each focused on a different style. The Guardian is focused on melee, the Consular on the Force, and the Sentinel on skills.
Jedi Guardian
The Jedi Guardian class focuses on physical combat and outlasting the enemy. In general, they prefer melee weapons. They get fewer Force points, powers and skill points per level-up than other classes, but get more feats and vitality points per levelup.
Related Prestige class: (Jedi) Weapon Master/(Sith) Marauder
Jedi Consular
The Jedi Consular is the Force-focused class, analog to the wizard in fantasy RPG's. If they're forced to take up arms, they prefer blasters or lightsabers over conventional melee weapons. They get more Force points and powers than guardians, a few more skill points, but less vitality points and fewer feats.
Related Prestige class: (Jedi) Master/(Sith) Sith Lord
Jedi Sentinel
The Jedi Sentinel is the all-rounder. He doesn't favor any certain type of weapon, and gets an average amount of feats, powers, and Force/vitality points. Where he excels is skills, which would lend them to less conventional means of combat, like laying mines, stealth attacks, repairing droids to fight for them, hacking turrets, and overloading conduits.
Related Prestige class: (Jedi) Watchman/(Sith) Assassin