Multiplayer in Mass Effect 3 is a co-op experience for up to four players. You’ll face wave after wave of bad guys, and you earn experience and credits for how well you do during each wave. Your experience is used to boost your abilities. For instance, I played as an Engineer, so as I leveled up I was able to improve my combat drone and powers such as Overload. Credits gave me access to bonus gear that I could use during a match, like being able to heal myself, gain extra ammo on the fly without going to a resupply point, and so forth. Gaining access to new weapons, better gear, and new powers makes multiplayer a compelling experience. 
Now, this being a demo, it’s hard to get a feel for just how much impact the multiplayer will have on your single player campaign, but Bioware has said it will make a difference. For me, that’s one of the most exciting possibilities. Playing things like Horde Mode for Gears 3 was fun, but to have it influence the campaign, for there to be some consequences in the game proper depending on how well you do instead of being a completely separate part of the game could be a really nifty idea, and will certainly compel some players to spend as much time with multiplayer as they do in the campaign. If the synergy between the two modes works, we might see this model copied by future games because the idea is just so compelling.
One of the great parts of multiplayer is it gives you the chance to try out the different classes. Be an Adept and try out using all kinds of biotic powers. Be a Soldier and enjoy using some serious firepower, be a Sentinel and try a little of everything and so on and so forth. You can pick your class, your race, customize your armor and more. All in all, there’s lot to explore with multiplayer, and obviously much more will be available when the full game arrives.
The biggest surprise of Mass Effect 3 could be the fact that the multiplayer might turn out to be really fun. 
