Online Multiplayer

From RWP Wiki

The Online Multiplayer feature of Xbox Live allows you to play with other people who are currently playing the same game as you. This is accomplished by players joining a game session that you started or joined. Some game sessions are private or invite-only, meaning that you have to be invited to join the session. Other game sessions may also be full, meaning that the maximum player capacity has been reached, and these sessions will reject your attempts to join until a player leaves (or gets booted from the session).

It is possible for a game to support Xbox Live, but not support Online Multiplayer. A good example of this is Tom Clancy's Splinter Cell, which allows you to download new missions, but does not allow you to play with other Xbox Live users.