Raiding is, essentially, scavenging during a fight. While it's usually smarter to loot a place after all resistance has been cleared out, that is sometimes not an option. Raiding is primarily used when players make gorilla style hit and run attacks in order to steal resources. Targets include trade caravans, defended resource deposits, or any bastion of civilization. For this reason, raiding is often practiced by the less reputable residents of the wasteland.
Running a Raid Edit
Treat a raid exactly like a fight, but whenever one player wishes he or she can sacrifice a turn to make a scavenging roll. You may require this to be made within a certain context. When raiding a caravan, for example, you should probably require characters to be adjacent to the actual caravan to search it for goods.