You can play this game together with friends, either in p2p mode or with a dedicated server (there are companies who rent full game servers, but you could also rent a bare metal server and host your own dedicated server on it). In this mode most people play co-op I would assume. p2p means you need good internet uplink speed and a better than average machine to host the game.
And then there are open servers (you will see a list first thing when clicking on the multiplayer option) with all kinds of rules, some only PvE, some PvP.