also, this is straight from
http://sheldonbrown.com/
"Singlespeed vs Fixed Gear
The one-speed revolution actually involves two different styles of bikes, singlespeeds and fixed gears. These are not the same thing, although they have much in common.
A fixed gear bike differs in that it does not permit coasting; when the bike is rolling, the pedals will turn, just as with a child's tricycle. To enjoy the one-speed experience to the max, a fixed gear is the best choice if you ride mostly on pavement. A fixed gear gives a degree of control and one-ness with the bike that is not equalled by a freewheeling bike.
This site also contains several articles dealing with Fixed Gear Bikes.
Fixed gear is not ideal for all circumstances, however. A fixed gear is not well suited for seriously hilly terrain, and, more importantly, is not good for technical mountain biking. A mountain biker in difficult terrain must be able to control when each pedal is down, to avoid striking a pedal on rocks, logs or other obstructions. Similarly, jumping over obstacles is much more difficult on a fixed gear. If your single gear is low enough for off-road climbing, it will be too low to spin on the descents.
You don't have to choose once and for all between fixed and freewheel, because the same bike can be both, if you use a reversable "flip-flop" hub.
Many cyclists interested in simplifying consider going to a singlespeed freewheel as a way to "test the waters" with the idea that if they turn out to like that, they might later convert to fixed gear.
This is generally the wrong way to approach it, in my opinion.
I STRONGLY recommend starting out with fixed gear. If it turns out to be a problem, you can easily convert to freewheel later if you want to...but my bet is you won't want to if you give fixed gear a good try (typically takes a couple of weeks of regular riding to get past the strangeness, but then it's quite addictive!)
Most folks who set up their bikes with a fixed/free flip flop wind up using the fixed gear side pretty much all of the time. The freewheel option is mainly useful for when you have taken a longer than usual ride, and need to get home even though you're all tuckered out. "