Raiders are dirty gang members spread out all across the wastland. They are very agrresive, and they like to spend their lives murdering, stealing, and raping. Except Children. Nobody hurts children. Never.
All kinds of buidings, sewers, and holes. Strangely enough, even though Megaton is the biggest hole in the Capital Wasteland, there doesn't seem to be any Raiders in there.
Their agressivity is caused by mutations caused by the high leves of radiation. Male Raiders are in a permanent state of adolescence and erection, explaining the violence and the rapings. The stealing might be due to cleptomania, or simply because they want to live to their namesake. Female Raiders are always menstruating, explaining the large ammounts of blood usually found in raider habitats.
For being a raider, you just need to go on raids. However, for being a Raider, you need to pass an initiation ritual (or a spell check, but this is unlikely, due to most Raider vocabulary beign formed of different grunts, shouts, and swear words). To achieve Raiderhood, the initiate is made to kneel on the ground with only his or her underwear in as a form of humilliation (ironically, the initiate may be wearing more clothes than the average Raider in that moment, but the Raiders have no sense of irony), with a croud of Raiders surrounding him or her. Then they start to insult and physically torture the initiate, but dealing with this is not the requeriment for passing the ritual; atracted by the crying and the shouts caused by pain, it is likely that The Lone Asshole will show up and shoot a Fat Man in the face of the initiate, killing him or her and the surroinding Raiders. If this does not happen, the initiate is considered an extremly lucky person and achieves Raiderhood. The Raiders dont seem to notice that these rituals usually cause their numbers to decrease rather than adding more members to their gang, but as said in brackets a few lines before, Raiders have absolutely no sense of irony.