Re: How do the different classes differ in playstyle
The wiki would be your best bet for an indepth answer. Game mechanics between 1, 2, and 3 differ and the classes take a drastic change between 1 and 2, with 3 turning it up to 11. Short of it:
Adept: force user and mini black holes.
Engineer: tech support at me1, summoner class at me2 and 3 with the addition of drone skills.
Soldier: shoot them till they're dead.
Vanguard: shotgun force user at me1, me2 introduced charge, a way to put the shotgun at an enemy's face. Me3 gave them nova for charge>nova>heavy melee combo.
Sentinel: force user with tech powers and tankiest class starting at me2 with the addition of tech armor skill.
Infiltrator: sniper expert at me1, gained cloaking in me2, got better at it in me3.