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Raids allow 2-4? parties to team together so that they can tackle even larger challenges. Such as monsters that con red^^ to your highest group member, and the level of the monster is 10 above that of your highest member. It gives you a larger group basicaly.
Correct as far as it goes, but there are a couple other things. Certain zones (Bloodskull Valley, Zarvohn's Tower in FP) are raid only, requiring 6-12 people. Other raids require (or allow) up to 24 members total. These raid instances are designed for the coordinated efforts the listed number of people (of an appropriate level of course) and generally net a greater reward than solo or single group content situations.
Raids can be done by anybody who wants to set one up, but in general you will find guilds doing raids more than just random groups of people. If you are interested in raiding, you may want to seek out a guild on your server in your level range and ask them about what form of raiding they do. You'll be able to get a much better idea of the specifics this way.
Good luck, and may you have fun on all your raids :)