GTA V is set in San Andreas and so will GTA Online.. at least only for now.. Other famous locations like Liberty City will be added to GTA Online sometime in the future (Vice City as well?!). This emphasizes the promise that more and more content will be added in the future to GTA Online.
That doesn't mean we fans have to wait for a while before we get the full experience of GTA Online.. With over 500 missions at the start, it will be a while before you even get bored. Some missions can be done alone while others will need you to gather a couple of friends. (That's why you will need a crew, come join Temasek Thugs, in what I hope will be the largest Singapore Crew.)
Almost all activities from the Single Player mode will be available, including the important ones: Robberies, heists, assaulting gang hideouts, hijacking vehicles & even tennis, golf and base jumping.
Many GTA fans have been clamouring for 2-player co-op modes in GTA. We want to play and Niko and Roman! Rockstar answer: Play as a crew!
GTA Online almost looks like a MMO from what I've seen of it so far.
So Good Bye Social Life or.. maybe I can meet with all friends online in GTA Online!
Source: IGN
