Quick guide to Midgard classes:
Tanks:
Warrior: high melee dps, great defense, main tank, can climb keep walls.
Thane: med-high melee dps, med ranged aoe, good defense with shield.
Valkrie: medium melee dps, good defense with shield, decent heals (point blank aoe and casted single), insta rezz, aoe snares.
Skald: the midgard speed class with top run speed buff, medium melee dps, limited ranged dps, mediocre defense, single target mezz.
Melee dps:
Berzerker: very high melee dps, can dual wield, mayhem, best in class.
Savage: same as berzerker but with short duration self buffs that are hard work to maintain. Zerker is better.
Mauler: no idea on this one, was an expansion class. Almost never see them about, so probably gimped.
Stealth:
Shadowblade: very high melee dps, can dual wield, can climb keep walls, uses poisons, reliant on evasion for defense.
Hunter: high ranged dps, bow for ranged and spear for close combat, also has a medium useful pet.
Casters:
Runemaster: very high single target dps, mediocre aoe, roots, bubbles, run speed buff. Solid class.
Spiritmaster: high single target dps, very high point blank aoe, has a very useful pet, mezz, basic rezz. Solid class.
Bondancer: high ranged dps, good survivability, relies on a myriad of pets to function, never quite worked this class out but apparently powerful.
Warlock: high single target dps, high point blank aoe, ranged dots. Uses a wierd system of pre-charged spell chambers. Gimped. Avoid.
Healers:
Healer: does what it says on the tin. High heals, great crowd control (mezz & stuns), gimp damage. Popular class, hard to level solo.
Shaman: can either be played as a healer and buffer, or as ranged dps with aoe dots, roots and diseases. Fun to play as Cave, usually relegated to a buffbot otherwise.
Note: all Midgard melee classes can swap freely between 1h and 2h weapons, or between dual wield and 2h in some cases. All casters get a quickcast ability (very useful given the interupt code in this game). Healers don't get quickcast.
All the Midgard classes are fun. Only ones I'd stay away from are Warlock (nerfed and has a horrible mechanic) and Mauler (seems a hated class). Everything else is good for a whirl.
Other realms have a similar spread of classes, though not mirrors (which is the beauty and the beast of Daoc).