By definition, the opposite of slow food. Duh.
Generally, a category of restaurant that serves food quickly at the possible expense of other considerations. The point of fast food is that it is delivered quickly, not that it is a unique and beautiful culinary experience. They are generally considered to be cheap places to eat, but reality often does not hold true.
The worldwide marketplace is held by a few major brands, a few more minor brands, and countless smaller chains and independent shops. These generally are American-owned, with only slight variation between countries.
Here's a list of the major brands:
There is something to be said for the E2 community that a working node for slow food existed since 1999, but this fast food node has been empty until 2003. I'm not sure what that says, really, but I'm sure it's something.