It doesn't but it does have downloadable content.
Mass Effect and Mass Effect 2 are both single player. There are no multiplayer modes
No. That's a single player game.
No
There is neither a co-op mode or online mode for Mass Effect 2
Yes.
If you want to, and you're into video games. Try reading a few online reviews about it.
Mass Effect 3 campaign is only single player in itself. You can however play online with other people by using it's multiplayer functions.
This is up to you. But, my opinion is that you should definitely play ME1 first. You get a better feel on the Mass Effect Universe, and you will be more attached to the game, and will enjoy ME2 a lot more. Mass Effect 1 is a great game, as is Mass Effect 2, but if you have the time and the money, play ME1 first.
No, the player can only play as a Human in Mass Effect 2. The player can, however, date non-humans.
You won't be able to import your save game from ME1.
If you don't import a save game from Mass Effect, you will automatically choose the character of the opposite gender to survive. If you play Mass Effect first, though, you can choose if you want Ashley or Kaidan to survive. So to play Mass Effect 2 as a female and with Ashley alive, you would have to play Mass Effect and let Kaidan die on Virmire.
Xbox 360, PC and PlayStation 3