Scavs do what they have to, which is looting. The Svavengers are partly enemy players and the others are controlled by AI. Next to that you have Scavengers that will shoot you on sight and are there to find as much stuff as possible. All the loot you find, even as other characters, can be used on your PMC to make him stronger. First of all, you got the PMC, which is your main character that you want to make stronger. There are three different options in the game. This game is special though, since you won't always start the game with the same character. You can then use this loot to improve your gear and weapons so you can jump back into the game as a stronger character, or to just sell all your stuff for in-game gold. If you succeed, you can keep all the loot you got during that session.
The goals seems simple: you choose a map, reach the extraction point, loot and survive while doing so.