You can find all sorts of technical breakdowns of what the different classes are, I'm sure. I haven't really looked. In fact at this point the only thing that I reference when trying out a new class is this page that a guild member linked me to. If you look at that page you'll see there are 6 classes. 3 classes can chose to be DPS (damage dealing) or Tank (damage taking) and the other 3 can choose to DPS or Heal (fix those booboos).
The Tank/DPS class choices are Warrior, Engineer, and Stalker. The Heal/DPS classes are Esper, Medic, and Spellslinger.
I'm using what's left of open beta to try out various classes and feel them out. Next to each class type I am noting the max level I've achieved on that class, since it does matter, as different abilities open up at different times.
Warrior (12): Warrior SMASH! At level 9 you you get Whirlwind, and it's seriously fun as you use attacks to generate Kinetic Energy (aka Angry Red Face) then unleash that energy to become a spinning sword of death. The R button ability puts you in Overdrive, aka you get swole and smash dudes even better.
Engineer (7): Big guns and robot minions, what could be better? I haven't leveled very far but I have one robot to help DPS down the bad guys, and another to help by tanking them so my insides don't get on my outsides. I have a really really big gun.
Stalker (14): Slash Slash Jumpity Slash! This melee feels very nimble and aggressive, quite different than the warrior. As someone who is map challenged I also appreciate the stealth, it gives me time to look around and get my bearings without ending up in combat every few seconds.
Esper (8): The spell effects on this class are really cool. Mind daggers and psychic bird attacks! The hardest thing to adjust to was not being able to use my main attack on the move. I've still been enjoying the class, and it is still highly mobile which is good given the importance of dodging enemy attacks.
Medic (3): This is not a class that I've managed to get into yet, though I am going to keep trying a bit. I think I might like it when I can try out healing? It is kind of hilarious to shoot electric charges out of my defibrillators though.
Spellslinger (4): This is the class that I understand and like the
least. The one probably leads to the other. I thought I would like
shooting spells from pistols, but not yet. The resource build up is
similar to warrior where you build up energy, but this time you hit R
when it's full to power up your spells.
I'm not sure how much more time I'm going to spend on other classes prior to launch. I haven't played my engineer since my first beta weekend, so I'll probably give that a bit more time. Right now I'm leaning heavily towards playing a Warrior or Stalker in live. I've had the most fun with those classes. I feel I should try a few more levels on the Medic and Spellslinger to see if a new spell really changes them, but on the other hand if I'm having a blast dicing baddies up on my Stalker, then why worry.