Originally Posted by
tmeisen
That and horizontal dropouts.
On fixies you need a means to adjust your chain tension; geared bikes (commonly with vertical drop outs) will use an idler mechanism, usually built into the rear derailleur. Since this isn't an option for fixies, the rear dropout is a slot so you can move your rear wheel fore and aft to adjust the chain tension.
You don't necessarily need horizontal dropouts to make a bike a fixie. The
White ENO hub will allow you to make a vertical-drop frame into a fixed gear, as will a
Phil Philcentric BB.