When it's time to go slay the dragon, you aren't going to do it alone. In fact, unless you're Beowulf, not even 5 people will be enough. That's when it becomes time to call in a small army to help you.
Motives for a raid can vary. Most players are there for the phat lewtz or for the thrill of killing a major opponent like Illidan. Some players participate for social reasons, or for a combination of all of the above.
A raid is formed from a group that wants extra people added. As a result, you must be in a party to create a raid. From there, the group leader needs to go to the raid window and select "Convert to Raid." At that point, the screen will change to a matrix of 8 groups, and additional players that are invited will fill in the earliest slot available. Since the raid is still a collection of 8 five-man groups, the leader can shift players around as he sees fit.
To make managing a raid simpler, the raid leader can assign players to be assistants, which allows them to invite others or mark targets. They are not able to change the loot method, but can rearrange groups.
There are several reasons why one might raid, but the most frequent is to complete specialized instances that are intended for large-scale battles. There are also a few outdoor bosses like this, as well as circumstances in PvP where one might string together multiple groups for communication reasons.
Raid zones include:
With the release of Burning Crusade, Blizzard decided to change the player cap on raid zones from 40 to 25. This means that a maximum of 25 players can zone into the Burning Crusade raid instances, (which begin on the list with Karazhan, one of the half-raid zones.)
Raids are typicially held either by a single guild or occasionally by an alliance of two or more guilds. Some guilds exist strictly for the purpose of raiding high-end dungeons. However, oftentimes groups of players that have already completed some easier raids will run a PuG raid against that boss, and in some high-end instances trash farming raids are common.
Since raids are much more difficult than basic five-man encounters, a successful raid will probably need all of the following:
Besides that, players will also need to work as a team as best they can. This means using the best buffs and debuffs to help their allies, using good consumables and gear, having the appropriate mods and sometimes voice chat as well as just following instructions and playing well.
After clearing the trash mobs blocking the way, hopefully a raid will be able to assail and kill one of the bosses. Bosses are incredibly nasty and have several devastating abilities that the raid needs to be aware of and knows how to deal with. Bosses can do all kinds of nasty things, including putting Status Effects on the raid, dealing AE damage, and summoning minions. Many fights are on a timer that forces the raid to finish quickly or the boss will enrage and start hitting much harder than before, usually leading to a wipe.
If a raid manages to down a boss, they will get two or three pieces of loot. Different raids will have different policies regarding loot, and these should be clear to everyone at the start of a raid. Many guilds use a DKP system for loot distribution.
The raid will hopefully be able to continue on to another boss before the night is over, but eventually the raid will have to be called. Experienced guilds may be able to clear a dungeon from start to finish in one night, but normally this will not be the case. In the event that a raid breaks up for the night, each player will be saved to the Raid ID, and will be locked out of joining a new group for several days. This reset time is fixed and depends on the dungeon. Most dungeons reset every 7 days, on Tuesday mornings during server maintenance.
When a raid is formed, players in it will not be able to make progress toward finishing quests, even if they are soloing in a far-flung corner of the world. There are a few special quests marked (Raid) that can be completed, but these are the exception.
It is not possible to un-make a raid. If a group wants to stop being a raid, they will need to disband and reform.
Raids have their own chat channels for communicating with everyone in the raid. The slash command for this is /ra. The raid leader's messages will appear in a different color to stand out. Raid leaders and assistants can also broadcast a message to every player, causing it to appear in the middle of the screen with a sound effect, by typing /rs. For ease of communication, many raiders prefer to use voice chat rather than type.
Guilds that form around the premise of raiding are usually expected to be well-oiled machines, with defined leadership and rules, and a clear expectation to ultimately progress through a dungeon and graduate on to the next one. Since each dungeon is harder than the previous one and requires the new gear from the previous instance, the word Progression is very important to many raiders, who do not wish to stagnate.
Joining a raiding guild often requires the ability to commit several nights per week to raiding, and passing an initial application period. Guilds have very different policies about how they treat new members, however the more elite guilds have very high expectations of their members in terms of skill and hopefully gear.
Players that raid often do so for superior equipment, and feel that their acquisition of this gear makes them superior...if not for the extra time spent on the game and skill and organization shown to complete a boss fight, then simply because of the gear itself. There is often a rift in terms of game development between raiders and more casual gamers. Raiding also represents the pinnacle of PvE accomplishment, and so raiders are often resentful of devoted PvP'ers when new PvP gear becomes available or a change is made to the game that seems PvP-oriented.