Diana Farrell, who is now a member of America's National Economic Council but until recently worked for McKinsey, a consultancy that has spent a lot of time studying the middle classes, reckons they begin at roughly the point where people have a third of their income left for discretionary spending after providing for basic food and shelter.
You begin with three classes and a number of other classes are available to unlock either through in-game experience points (earned during matches) or by paying cash.