Raiders bases
Raiders live in two bases across the wasteland. One is a cave in which their main hideout, a "really secret spot," is located. The other is just a small outpost in the south, for sometimes they have some operations to do in that area.
The main base
The main base of the raiders is the one you can encounter in Fallout 2. It has only one entrance, and you can go there only by completing the quest in the southern outpost first (See below). The outside part consists of small houses, tents and pens, where most of the raiders live and wander around. A small, heavily guarded entrance in a cliff leads you to the cave of the raiders, where they keep their supplies and their special Workbench with additional tools. This is also where the leaders stay so feel free to talk to them if you want. Here you can also resign from the Raiders faction. You can also trade there with a drug dealer and weapon&armor dealer. You can bring ranger dogtags to Gomes , he is located outside of cave, for now dogtags gives only reputation ( +4 rep. each).
The southern outpost
The Raiders' southern outpost is located near Vault 15 and is ruled by Jim Raynes. It is basically a small, not really secret, outpost where the newbies are sent to be tested. In this location you will do your first quest for the Raiders (killing Cooper). Only a few raiders stay here permanently to watch over the new members, gather supplies, or send messages to the main base.
Very similar to Fallout 1's Khans base, this outpost is only used as a small hideout: "If you've ever doing work in that area, then this is your safehouse", as Raynes tells you. He even adds: "Yeah, it's basically just a crash house for the boys if we hear about a juicy target from the Hub or Junktown. When there's nothing going on, the few of us that stay here all the time mostly terrorise homesteaders for what we need".
They might have some jobs to offer you there if you have joined the Raiders faction (Jobs not yet implemented in the game though).