A raid is a type of mission, where the objective is to use a very large number of people, relative to a normal team size set by the game, to defeat a boss monster. Raiding is large-scale combat (of usually two more more in-game groups) against a common enemy; one source says groups are of 5-20 people. There will be some pre-level 50 raids. There will be tiered raid progression.
Usually a boss has enough defenses that it would be impossible to defeat it outside of the designed raid size parameters for the particular boss. For example, a normal team size in a game may be 5 people, but a raid for a particular boss may be up to 20 or more people. A normal team size wouldn't be able to handle the level of damage that the boss can do, therefore it is necessary to have enough people so that the damage dealt by the boss is manageable. This can be done through a large enough group of healers and other damage negation classes. Another benefit of an extraordinarily large group of people are stacked buffs, where the damage dealing classes are buffed, through an increase in damage or accuracy, by other classes such that the damage dealt to the boss is enough to kill it.
Character types typically represented in a raid:
Item drops from raids are often very rewarding, including unique or maximum stats items, thus giving players of the game an incentive to participate. Other rewards may include large amounts of experience and money. The high rewards often come at the cost of high risk to the player. The large number of people in a particular raid group inherently increases the chance of any individual executing an action (or lack thereof) that would be detrimental to the raid as a whole, up to and including raid failure. It is therefore important to have a strategy to defeat the boss before starting the raid. A raid leader is often needed to control the group due to the complexities of keeping many players working well together.