Your job is to get beaten upon by mean nasty monsters while keeping everyone else from getting hit. This role is usually played by a protection Warrior, protection Paladin, or a feral Druid. Tanks should be heavily armored. Their spells and abilities should be geared toward maintaining "threat" or "aggro."
The threat/aggro metric determines how angry a monster (mob) is at you and determines how likely it is to attack you. The tank will use taunt and high threat/aggro abilities to keep mobs hitting them. Other players can pull threat/aggro by healing, doing large amounts of damage, or using high-threat abilities.
The main function of the healer is to keep the group from dying. Their primary target should be the tank as they should be taking the vast majority of the damage. Priests, Druids, Shamans, and Paladins are prime candidates for the healer role. Most healers are Holy or Restoration specced. Be prepared to resurrect (rez) fallen comrades. In the event of a wipe, where everyone dies, make sure you raise other classes with resurrection abilities first so you can get a little help with this process.
All characters should come prepared for the instance, but healers most of all. Be sure to bring along plenty of water form mana regeneration between pulls. Also make sure you have the highest level of mana potions for your level and reagents for spells like Ancestral and Rebirth. A Warlock's Soulstone should be cast on someone in the group with the ability to rez the party.
The role of the rest of the group is to kill the mobs. Every class has specializations for dealing damage. While it is very tempting to go all out on your target, you are responsible for managing your own aggro. Your threat level should not be higher than the tank's, lest you get swatted by the monsters instead. In most circumstances, DPS should wait until the tank has started combat and built aggro before attacking.
Most of the time area of effect (AOE) abilities are not appropriate in an instance. By doing AOE attacks, you may raise your threat levels of many monsters, and may cause them to attack you instead of the tank. Also, AOE may break crowd control measures, which may create an additional burden on the tank and healer.
Many groups rely on crowd control to lower the number of active mobs in group pulls.Be careful not to break CC out of turn, he tank should break it at the appropriate time.
Mages are often called upon to Polymorph or "sheep" a target. This is a renewable form of CC, as the Mage can and should recast the Polymorph spell if the group is not ready to DPS their target before it breaks.
The Hunter's Freezing Trap is also a renewable form of CC. Be prepared to cast another trap as soon as the first one fades. In some cases Hunters may need to start combat by shooting their Freezing Trap target and then casting Feign Death to drop to the bottom of the aggro list. This allows the tank to take over.
Rogues will be called upon to Sap targets. Rogues should be able to Stealth forward to their target and Distract them before performing Sap on their target. The rest of the mobs usually ignore the rogue and then the combat is initiated as normal.
A Note on Raid Icons
The party leader has the option of using raid icons on the monsters. Although conventions on what each icon represents vary from group to group, there are some common conventions:
- Skull: Primary DPS target
- Red X: Secondary DPS/Sap target
- Moon: Polymorph target
- Blue square: Freezing Trap target
- Purple diamond: Mind Control/Shackle Undead target
Many classes have ability to buff their party members, giving them special abilities that help makes things run smoother. You will most certainly have learned these, but remember to cast them at the beginning of the instance, as they fade, and any time a party member is resurrected. Mages should be prepared to share conjured food and water with their parties as necessary. Shamans should be sure to drop appropriate totems as they act as party-wide buffs.
Instances can be fun and frustrating. They work best when everyone knows their role and plays it to the best of their ability. Make sure you are prepared with potions, reagents, food, water, ammunition, and plenty of bag-space. You may be offered "run throughs" by higher level guild members. While this is helpful, and gives you a quick jump in experience, practice your appointed role whenever possible. Once you cross through the Dark Portal, you will have to carry your own weight in a party. There is nothing worse than someone who doesn't know how to play their role.
Edit: Thank you for all of the wonderful comments and discussion. I did forget to add in some cc abilities like Hibernate and Banish. One last thing: Hunters, unless your pet is off tanking or you don't mind your fuzzy wuzzy being squashed into oblivion, be sure to turn off your pet's growl ability.