Gamemodes drastically effect gameplay by introducing new objectives. There are 5 gamemodes:
Just player vs player and no monsters.
Team vs team and no monsters.
Free-For-All (aka PvPvM)
Player vs player vs monsters.
Player vs Monster Coop (aka PvM)
Players vs monsters and everyone is on a team.
Team Deathmatch with a twist. Randomly around the map brains appear. Pick up enough brains and your team wins that round. Special monsters spawn that give more EXP. This gamemode is only playable when there are at least 6 players in the server. This gamemode will level you up quickly.