Fixed gear means that the rear sprocket is directly fixed to the back wheel, so whenever the wheel is turning, so are the pedals. Early bikes were all like this, and when the ratchet mechanism freewheel was invented it was hailed as a great innovation and became universal very quickly and led to the single-speed.
 

Back in the good old days, club cyclists would ride fixed in the winter as fixed is good on ice. Then cycle couriers started to ride fixed, which is fine, as fixed is reliable and efficient.
 
The cyclist in the picture above is an example of the kind of person who should ride fixed. He's very experienced. The other chaps in his club have told him all about what riding fixed means. He has a nice old Sun bike, and has left the back brake on. The wheels are 27", with 32 spokes in the front wheel and 40 in the rear, the way it should be. He's wearing black, a habit from his days in the British League of Racing Cyclists (or even the LCP). He has his cloth hat on backwards, and he didn't pay for it, no he found it by the side of the road. He has crochet backed track mits on. When actually riding he will bend his elbows, as his natural graceful riding position has been compromised by the need to pose for the camera.
 
With fixed, you only get one gear ratio, unless you can find a Sturmey Archer ASC, which is a 3 speed fixed hub. SA will start making these again, and when they do we will stock them.
 
HOT NEWS: The Sturmey Archer S3X hub arrived at Maryhill today, 8/10/09. This is the new 3 speed fixed gear hub. We have it fitted to a bike and will be seeing if we can break it.
 
Report 16/10/09. The S3X is lovely, and we have already taken one order. The price is £250 for the hub, sprocket and control. A rim and wheelbuilding will be about £45, and the rest of the fitting will be from free to lots, depending on what the bike is like.
 
Please note that only the bridgeton branch sells single speed and fixed gear bikes. Maryhill will sell 3 speed fixed bikes.