How do I change my single-speed to a fixed gear? I've been biking on single-speed for a few months and would like to play around with a fixed gear. I've heard of a flip-flop hub, but I'm not sure exactly what it is. And without any fellow bikers - I'm completely lost.
Benzo and Glenn Gervais are right on, but I thought I'd include a photo for any visual learners. This is a typical fixed/free, high flange rear hub. Quite often they're available in 120mm and 130mm OLD to fit different width dropouts. These hubs generally have solid axles without quick releases to prevent the hub from slipping and slackening your chain.

The right side is for a fixed cog. (The hub in the picture has a lockring installed already). The left side is for a single speed freewheel. 

Below is another example (a White Industries ENO Eccentric) with two big differences. First, instead of two opposite-threaded areas for a cog and lockring, this hub has splines for a splined cog and threads for the lockring (left side of the picture). The second difference is the eccentric axle which allows for chain tension adjustment on bicycles with vertical dropouts (single speed or track specific bikes have horizontal or rear-facing dropouts for chain tension adjustments).