Raiding groups (also known as raiding communities) are groups organized by regular players. Usually their purpose is to recruit other players and then venture into end-game instances which require raid parties, like Naxxramas, Archavon, Sartharion, and Ulduar of course. They are not necessarily related to guilds in any way, shape, or form, but are simply an organized group made by and for any able, committed max-level player.
Such groups are usually formed through the merging of two or more guilds, or through the simple recruiting via the World of Warcraft realm forums, or in-game advertising. The progress and value of the effort given within these communities is usually just as good as any guild's.
Nearly all raiding groups use some kind of formal Loot system, usually a form of DKP.