Warrior: High HP, Heavy Armor. Can be very defensive (1h) to very offensive (2h)
Rogue: Str, but also DD or CE for ultradamage. Xbow has range.
Wizard: Int, high dmg, ultralow defense.
Warlock: Casts spells and has a shield. DoT skills and in my opinion the most fun-skills in SRO.
Bard: MP support. Lack of offensive kills.
Cleric: HP support. Lack of offensive skills.
You can mix all classes to get a, to you nice, balance.
Or not balance if you feel like it

Most seen: Warrior/Cleric, Rogue/Bard,
Wizard/Bard, Warlock/Cleric.
Of course a lot of other combinations, but just like I've mentioned, do whatever you like.
THIS IS ALL IMHO.