Too bad they're removing nurse cheese, sucka
There are four classes. Each one suits a different playstyle.
Melee: Deals the most damage up close. Has the highest defense. Easy to Learn, Easy to master.
Ranger: Deals incredibly high damage and is highly adaptable. Has the highest consistent damage. Easy to Learn, Hard to Master.
Mage: Has the coolest attacks which are easy to use as intended, but is very hard to play effectively. Hard to Learn, Hard to Master.
Summoner: Summons minions and then uses weapons from other classes. Generally the rule-breaker class. Has the highest theoretical damage and is even more adaptable than ranger. Hard to Learn, Easy to Master. (It's not very intuitive for most, but it's easy to get the hang of once you DO learn it.)