BioWare Details Mass Effect 3 Multiplayer Galaxy at War Feature
The Galaxy at War mode is an optional multiplayer experience that Mass Effect 3 players can use to help influence the war against the reapers by playing together and increasing the Galactic Readiness of their single-player games. The single player experience will remain on its own and seperate from the Galaxy at War mode. Rather than playing with established characters, players will get to create their own characters from a number of different Mass Effect races such as krogan, turians and asari, each with their own unique abilities. Players will be able to level up their created characters and progress with weapon upgrades.
The key to victory in the single player campaign is the Galactic Readiness level, which players can achieve the best ending with without the use of the Galaxy at War feature, but if they choose to take on some of these missions, it can increase their Galactic Readiness and aid them in the single player campaign of Commander Shepards final journey.
BioWare notes that the system is completely optional for Mass Effect 3 players and is not required in any way for the single player game, but it does provide the option to enrich things with a multiplayer experience.
Source: Mass Effect 3 Forums