Help! Explain guilds and raids to a noob

So basically both are good. It's not necessarily like one is better than another. But you'll hear people say on these forums that "higher tier raids are better and give better rewards". Well yeah for "higher" level players not lower level players. The character roster of high level players is more powerful than the roster of lower level players. So that why the higher level players can post more damage in a raid than the lower ones. Because of more powerful characters AND more powerful character rosters.