This game uses the aggro system common to quite a few MMORPGs.
Basically, how it works is that mobs are too stupid to choose targets in an intelligent manner likely to win battles. Instead, they attack whoever does the most "aggro" to them.
If you damage a mob, it does some aggro to that particular mob, but not to any other. If you heal, it does a little bit of aggro to all mobs.
When it is time for a mob to attack, it targets whichever target has done the most aggro to it, without regard to whether attacking this particular target will help win the battle.
Many games divide player classes into tanks, healers, and damage dealers. A tank is very heavily armored, so as to not take much damage when hit. A tank might get some skills to do lots of aggro, but generally can't deal that much damage. A healer can heal other players, but is easily killed and can't do very much damage. A damage dealer is fragile and cannot heal, but can deal a lot of damage.
The basic idea of combat is that a group has some tanks, some healers, and some damage dealers. The tanks use their super aggro skills to get mobs to attack the tanks, and not do very much damage because the tanks have very heavy armor. The healers spam heals on the tanks, and don't really need to heal anyone else much, because the rest of the group isn't taking much damage. The damage dealers kill the mobs, but have to be careful not to do more aggro than the tanks, or else mobs turn away from the tanks and kill damage dealers instead.
Some people like that setup. Mostly it's a recipe for having an awful time trying to find a group, because everyone wants to be a damage dealer. Most of the players who will tell you they're a tank and many supposed healers will ignore their role and just try to deal a lot of damage, which gets other group members killed.
The nearest thing that this game has to tanks, healers, and damage dealers is ice, life, and storm wizards, respectively. It doesn't entirely follow the complicated setup of some other games.
I never particularly liked the aggro system. Too often it leads to Bob the Really Big Dragon pounding on a tank for the duration of a several minute battle, while ignoring the healer who is spamming heals on the tank, even though the healer would die in three hits. That always struck me as mobs going through the motions of trying to be vaguely threatening before dying, rather than actually trying to kill you.
I guess it's a step up from the common setup of, a mob will follow a fixed route, and if you don't get in its way, it won't bother you. Guild Wars gave mobs a more intelligent approach of attacking whichever character will die the fastest.
Here, with no notion of spacing within a battle, that would only lead to mobs focusing fire on one particular player whenever you group, which would mess up the game if there's no way to counter it. Pacify and taunt are kind of your counters, I guess. I'd probably prefer a system in which mobs chose targets at random, though.
To give you an example, I have a level45 ice wizard who has almost 3000 hit points and damage reducing equipment. When I play with my cousin (a life wizard), I usually cast taunt on the monsters so they attack me instead of him (doesn't work all the time though). He then focuses on healing me while I focus on attacking the monsters. With this playing setup, we both stand a better chance of surviving combat encounters within the game.
When my life wizard cousin plays alone, he summons a pixie minion (who like healing themselves usually) then casts pacify on the monsters so they attack the minion instead of him. Giving him time to build up a big finishing move on the target monster. Take note that pacify also, doesn't work all the time.