Multiplayer games in Dead Space 2 are objective driven. Each match is divided into two rounds. One team plays as the humans and the other as the Necromorphs during the first round. Then the teams switch sides for the second round. A match combines the scores for both rounds so each team gets a chance to play as both sides.
When playing as the humans, you begin with a single objective and a time limit. Your team must complete that objective before the timer runs out or you lose.
Once you complete an objective, you will receive a new objective and additional time. In order to win a round, the human team must complete all objectives within the time limit. The Necromorphs' only objective is to delay the humans and prevent them from completing their objectives.