What makes a bicycle frame a "fixie frame"
 

I'm looking at basic frames and need to ask - what makes a fixie frame a fixie frame and a geared frame a geared frame?

Ex: Street Dog Gunnar. It calls itself a fixie frame. What does that mean?

"Fixie" hubs (and single speed) are generally 120mm OLD. Modern road hubs are 130. So, the main difference is the rear dropout spacing. If you really want a "fixie" don't get sucked into thinking you need a fixie specific frame. I'd guess that most "fixies" you see are a conversion using an old road frame, either with 126, or 130 spacing originally.

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.

+1
Horizontal dropouts or rear facing track-ends to allow fore-aft movement of the rear axle for chain tensioning.

Many "fixie" bikes are track frames which have no holes for brakes. When used on the road, these may be illegal and are a hazard. You should always use at least one proper brake on a fixie in traffic in case your chain breaks.

+1 Nothing worse than trying to stop in traffic using the Fred Flintstone method.

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.

A fool and a hacksaw/grinder.

A fixed gear track bike is by definition a fixie, but fixie is more of an urban style the incompasses more. A fixie frame is really any road bike frame that allows you to tension the chain without the use of a derailleur or any other device that bolted onto the drivetrain; ie horizontle dropouts, trac forks or EBB..but not any device that bolts on a derailleur hanger.