There are generally 3 main archetypes of characters in World of Warcraft; the Tank, the Healer and the DPSer.
Tanks are invariably heavily armoured, high health characters with the ability to dodge, block or resilient in some form or another to incoming melee attacks. This allows them to stand at the front and be a shield for the group whilst everyone else can safely attack. Tanks are normally paired up with a healer so they can last for as long as the healer has mana.
Healers are usually low armoured, low health, high mana characters with the ability to heal others over time or in one-shots. They normally stand at the back and are paired up with a tank.
DPSers are those classes that are rated for their Damage Per Second (hence DPS). Most classes fit into this category in form or another. DPSing is insanely easy. You pick a target, you use an ability and you cause damage.
For most of WoW, there is a very singular course of gameplay. As a player is trying to kill the opponent as fast as possible, there is an onus to cause as much damage as possible so that the monster can be killed before it can do its damage. Unfortunately, this doesn't apply to Instances, Dungeons or Raids, and here's the reason:
Inflicting as much damage in as short a time as possible will cause the monster in the dungeon to come after you. Unless you are over-geared against the monster, you will die.
I cannot stress this highly enough.
DPSers are generally people wearing anything up to Mail armour and usually they've sacrificed the ability to shrug off damage to gain the ability to inflict as much damage as possible. They cannot tank, and if they try to, they will die. Usually, this starts a chain reaction in which everyone tries to speed up damage rather than let the tank take over, and inevitably one by one, they die too.
Equally, but rarely, healers sometimes commit themselves to healing everyone in the team rather than just the main tank. And this also causes the team to die because healing more than one person generally means they're going to attract the attention of the monster. Healing the Tank is both health-efficient and mana-efficient; the Tank can take damage, he can block, dodge, etc.
Other players are not designed to take damage so their health will drop quicker, and that will cost more mana in the long run.
I've said all this because in my runs to dungeons, players invariably try and do as much damage as possible and when the team dies because they've attracted too much attention, the Tank is generally blamed.