Raiders

There are three distinct raider groups referenced by the first Fallout: Jackals, Vipers and Khans, although only the latter appear in the actual game. Quoted from the Fallout Bible:
 * The Jackals: The first clan, the Jackals, is your typical group of crazies. They have no morals except one: survival. They use group tactics to overmatch their enemies. They are craven cowards, though, and will not attack unless they know they can win. They band together in their hideaway and fight over the spoils.
 * The Vipers: The second clan, the Vipers, are mysterious followers of an ancient religion (or so they claim). They usually only come out at night to hunt for food or to conduct raids. They are very ruthless when it comes to combat. They prefer stealth to strength. They usually carry bone knives dipped in Pit Viper venom. This poison, when in the blood stream, paralyzes the victim. Most victims captured in this way are taken back to their hideout.
 * The Khans: The last group, the Khans, is probably the most dangerous. They live the lifestyles of Mongol warriors, raiding towns, burning what they cannot take and capturing the survivors for use as slaves. They usually travel in small scouting bands, but sometimes they roam as full war parties. The Khans above all else respect strength. They are eager in combat to prove their worthiness to the clan by engaging in hand to hand combat with fists or clubs. The Khans carry very few firearms (since they are for cowards). Anyone showing superior strength is worthy of their respect. The leader of the Khans is so because no one has beaten him in combat.

There are numerous other groups of raiders in Fallout 2.