Single speeds are just plain cool. The simplicity is what got me hooked to them. A flip-flop hub is when you can have a fixed gear on one side (the pedals are always moving), and a free-wheel on the other side (you can coast, and of course will need brakes to stop). Most of the single speed/fixed gear bikes these days will already have the flip-flop rear wheel, so you'll already be set. Just be sure to look to see if the model you are wanting has this. In your price range of around $800 you will get a nice bike.