There are two PvP and two PvE Multiplayer options and they are accessed from Nitro in the HUB.
PvP
Duel
In this mode you engage in combat one on one agaist another Shadowgun. These battles are short lasting no longer then five minutes and the one that registers five kills first is declared the winner. If after five minutes nobody has reached the required five kills the player with the highest score is crowned the winner. At the end of the match there is a loot lottery that awards a random reward to each player.
Ascendancy
This is a 4v4 player team deathmatch where you pick up the trophies that killed players drop. The first team to pick up 20 trophies is declared the winner. To keep trophies out of the hands of the other side you can pick up the trophies that your own team drop and you can also revive fallen comrades by standing over their graves and interacting with them. If neither side has the required 20 trophies to win the match when time runs out the side with the highest amount wins. At the end of the match there is a loot lottery but unlike Duel mode you are not guaranteed to walk away with anything. In this mode you will also increase your fame when you kill your foes.
PvE
In the PvE modes you will be teamed up with two other players.
Dungeons
These are large levels and to complete the puzzles in them will require your whole team to work together. There are two Dungeons in the game.
The Voltaic Fist:
This is the first Dungeon that you will unlock and the objective it to reach Regis The Voltaic Fist and defeat him.
Blade Dancer:
In the second Dungeon you unlock you have to face and defeat Riksh The Blade Dancer.
Arena
This feature is basically two horde modes where you and two other players face off against five waves of increasingly difficult foes. The last level is a boss which will require teamwork to defeat.
Bronze Arena:
In the first of the two arenas you will face 5 waves of the enemy that will become increasingly more difficult to defeat.
Silver Arena:
The second of the two arenas is unlocked at level 20 and will contain tougher foes.