Raid Groups
Raid groups are groups of more than 5 people and are typically used to defeat unique challenges at higher levels. In WoW, up to 8 seperate groups can be linked up to form a super-group of 40 players and such a group is called a raid group.
Raid group members cannot earn credit towards most non-raid difficulty quests, or more specifically, the ones involving killing creatures and collecting items. Raid groups also grant substantially less experience for defeating monsters than normal groups. However, raids allow players to overcome challenges that might otherwise be nearly impossible.

What this means is that most of the quests in the game including the ones involving group instances cannot be completed simply by zerging them with a huge number of players. To be more specific, quests involving group instances must be completed by no more than 5 players since raid groups do not earn quest credit and neither can players bring other players into their own instance if they are not grouped. (Players in the instance which are not in the instance creator's group are automatically teleported to their bind point after 60 seconds.) This is done so that the group instance quests would not be trivialised by numbers.

Raid Instances
Raid Instances are indicated by their red swirling portals at the entrance. These instances are designed for raid groups of anywhere between 20 players to 40 players.

The enemies in a raid instance are designed to be extremely difficult and require a raid group of experienced players to take down. Raid difficult quests with rare loot goes along with these instances and this prompts for a good amount of communication and cooperation among all the participating players.

In fact, some of these raid instances are so difficult that they require previous raid instances to be defeated allowing players to properly equip themselves for the next raid instance. Some of these instances also require "keys" to be obtained from various players and group instanced dungeons before players can even enter them.

When the final boss of an instance is defeated, the state of the raid instance along with the names all participating raid members are saved. For the next few days (length varies from raid instance to raid instance), these players would not be able to participate in the same raid instance again. Any attempt to enter the raid instance for these players will result in that player entering the copy of the raid instance in which the final boss was slain. Raid instances do not follow the rules of group instances which resets once 30 minutes have passed with nobody in it.

The following is a list of raid instances currently available in the game listed with the recommended level range and raid sizes they are suitable for:

Name Level Raid Size Zone Status
Onyxia's Lair 60 40 Dustwallow Marsh Contested
Molten Core 60 40 Blackrock Depths Contested
Zul'Gurub 60 20 Stranglethorn Contested
Blackwing Lair 60 40 Blackrock Spire Contested
Ruins of Ahn'Qiraj 60 20 Silithus Contested
Temple of Ahn'Qiraj 60 40 Silithus Contested

More raid instances will be added in the future.

Raid Calendar System
The raid-lockout system causes all instances to reset at a certain server time, regardless of when you were actually saved to the instance. The reset schedule is as follows:

Molten Core: Every 7 Days, resetting during weekly maintenance
Blackwing Lair: Every 7 Days, resetting during weekly maintenance
Onyxia: Every 5 Days
Zul'Gurub: Every 3 Days
Temple of AhnQiraj (40-man): Every 7 Days, resetting during weekly maintenance
Ruins of AhnQiraj (20-man): Every 3 Days
All resets will occur during off-hours, when the least amount of raids are active.

Battleground Instances
Battlegrounds are easily identified by their red swirling portals, and provide an opportunity for everyone to particpate in evenly matched PvP battles. Battlegrounds are available on both PvP, PvE and RP servers.

Read more about Battlegrounds.

Common Instances
There is exactly one "instance" in the game where everybody in the world enters the same single instance (so it is not really an instance anymore since there's only one copy of it all the time) and that is the Deeprun Tram that runs between Stormwind and Ironforge. Players of both factions entering this white instance portal will all end up in the same single zone, grouped or ungrouped. Quite obviously, the Deeprun Tram zone belongs to the Alliance faction.

To recap on Instances
Only those in a party can enter the same instance.
If no one in the party is in the instance for approx 30 minutes it will reset.
If you die you will resurrect as you re-enter the instance regardless of where you died (unless another player ressurects you).
You can still talk to people not in your group while you are in an instance.
There are minimum level requirements on all instances.
You can do an instance as many times as you wish.
If you leave a group in an instance you can't complete quest objectives until you rejoin that group.