A marauder is a person — or sometimes an animal — that moves around looking for something to seize, smash, or kill. The verb behind it is maraud: to roam in that raiding, hunting way. Marauder is the one who does it; marauding is the adjective you put in front of a group (a marauding gang; marauding elephants in the fields). It is a rather old, slightly story-book word: you meet it in history writing, news about armed bands, and nature films, more than in talk about a pickpocket. A thief who quietly steals a phone is not a marauder. The word wants movement, a search, and harm, not one small steal. Bandits, raiding soldiers, and hungry animals are the usual pictures.