Raiding Groups
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 usually require raid parties, like Molten Core, Blackwing Lair, Ahn'Qiraj, Zul'Gurub, and kill Onyxia of course. They are not necessiarily related to guilds in any way, shape, or form, but are simply an organized group made by and for any able, committed level 70 player.
Formation
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.
Progress
The progress and value of the effort given within these communities is usually just as good as any guild's.
Loot
Nealy all raiding groups use some kind of formal Loot system, usually a form of DKP. Normally the first instance where such a system is used is Molten Core. The main purpose of all loot systems is to allow members to be eligible for loot based on attendance.