Warriors are your grouping tank. They mitigate damage the best. They hold agro on names better than any other class in the game. Anyone able to "steal" agro from a warrior doing his job did so by trying to. With mercs now in the game, you can solo warriors carefully. You should note however, warriors tend to be out-tanked by other plate classes until the warrior hits level 70+. Up until that point warriors are kinda gimp at holding agro. After level 70 a warrior will have item effects, hate discs like crazy, hate augs, epic effects and some serious speed at hitting the mobs to get the weapons to proc.
SK's are nice solo toons but their dps sometimes suffers due to their mixed class (necro / knight). I am not overly familiar with this class other than to offer up that this class makes a nice #2 agro toon in a group with a warrior present. Their harm touch is sweet in fights and can take down a name even after you're dead or FD.
Pallies are heavy duty dps on undead mobs. They can heal themselves but not to the point that it worth noting at higher levels. Again I don't know much about this class either other than their epics suck (LMAO - buggy and heavy grind)