There are three roles in a instance or raid , tank healer and DD.
All class can be a dd, but not all class can be a tank or healer. All class have three talents beside druid, druid have four talent.
In level 80, DK(death knight) can tank or dd in all talent, but it changes in level 85.
| class | talent and the roles in a instance |
|---|---|
| mage | arcane (dd) fire (dd)frost (dd) |
| priest | discipline (healer) holy (healer) shadow(dd) |
| druid | balance(dd) feral(dd) guardian(tank) restoration(healer) |
| warrior | arms(dd) fury(dd) protection(tank) |
| warlock | affliction(dd) demonology(dd) destruction(dd) |
| rogue | assassination(dd) combat(dd) subtlety(dd) |
| hunter | beast mastery(dd) marksmanship(dd) survival(dd) |
| paladin | holy(healer) protection(tank) retribution(dd) |
| shaman | elemental(dd) enhancement(dd) restoration(healer) |
| death knight | blood(tank) frost(dd) unholy(dd) |
| monk | brewmaster(tank) mistweaver(healer) windwalker(dd) |
As you can see, there are 5 class can be a tank (after level 90), and 5 class can be a healer (after level 90), and only 3 class have tank and healer talent.
In some special situation, dd can be a tank when raid boss have special skill, but this situation almost disapper, Blizzard do not design it again (after level 80).
There are need 2 tanks and 3 healers in a 10 raid , and 3-4 tank and 5-6 healer in a 25 raid normally before level 80.