These are the casters. They augment their magic abilities with technology, however. They both are ranged DPS and can add fear as a skill.
Summoner: Caster with multiple pets. Though the Summoner can deal decent damage, the pets are really the ones doing most of the work. Unlike the Warlock class in WoW, the Summoner does not do much to control the pets and can have quite a few at a time, depending on which skills are chosen. This is a great class for learning because while you are trying to get your bearings in a new place or busy ooh and ahhing over the really cool monster you're fighting, your demon pet takes all the damage and your fire elementals DPS the mobs down. If you do manage to get hit, you can heal from corpses.
Evoker: The Evoker is your typical ranged caster: powerful, fun, squishy. The Phase power is a particularly useful one as it decreases the damage output while increasing the damage taken by the target.
These are your gun wielding, ranged DPS classes.
Marksman: This class specializes in extremely powerful ranged AOE attacks. Running around with my husband's Marksman means that I'm stepping in big puddles of fire all the time and not getting a chance to do much damage. As long as you are good at keeping your distance and at figuring out where your grenades are going to bounce, mobs die quickly with this class.
Engineer: The Engineer shares many of the skills with the Marksman but also has pets. From level 1, the pets debuff the mobs so that the Engineer can DPS the mobs down before taking any damage. More pets can be added to the bot army very quickly as the Engineer levels. This is another great newbie class for survivability, though it is also considered extremely powerful in the upper levels. Like the Summoner, the Engineer is unable to have much control over the pets.
I found the Evoker class the most challenging, but it still has a lot of fun potential. Overall, the game is very easy on Normal Difficulty, however, and I think any of the classes are viable for playthrough. And though I've gone to the trouble of describing the classes for you, I actually recommend trying each class out for a couple quests to determine your favorite. I don't normally play a Warlock or Magician, so I didn't expect to like a caster/pet class as much as I do.
What class seems most interesting to you? If you are already slaughtering undead and demons with me in the subways of Hellgate: London, which is your favorite class?