Some people use a singlespeed as their "around the neighborhood" bike, when you don't need to get all dressed up in your spandex and cycling shoes, but just want to grab-and-go for quick errands, a casual spin with the kids, or on easy, flat terrain like the Towpath or other bike trails.
Many people use a singlespeed as their "foul weather" bike, for commuting or other situations where they want to get out and ride no matter what the conditions, but don't want to muck up their "good" bike. The singlespeed drive train is simpler and quieter, with less moving parts, and less potential for something to break or become out of adjustment.
This simplicity also speaks to something more primal in some rider's minds. Without having to think about shifting, they connect with the road or trail in a deeper way, conserving their momentum, feeling the terrain, appreciating the freedom of movement that they had when they first learned to ride as a child. "Free your mind, and your feet will follow," some say. From my own personal experience riding a singlespeed mountain bike, I've found that I end up riding a little more aggressively and faster overall, because as I am cruising along on descents or flats, I tend to ride my brakes a little less, knowing that I need to conserve that momentum to help me up the next climb.
Most singlespeed bikes still give you the ability to coast. A singlespeed that does not allow you to coast is called a fixed-gear bike. With a fixed-gear bike, if the bike is moving, your feet have to be moving, too.
The most obvious difference in the design of singlespeed bikes compared to geared bikes is, of course, the absence of shifters and derailleurs. With a geared bike, the chain has lots of "slack" to allow it to wrap around all of the smallest and largest gear combinations. The rear derailleur's spring mechanism moves to take up the slack when less chain is needed.
With a singlespeed, there is no derailleur to take up slack in the chain. It's impossible to cut a chain to the exact length needed for the gears, so the frame must be designed to allow for adjustment to the chain tension.
Surly Steamroller track frame with rear-facing dropouts
Another solution to the singlespeed chain tensioning issue is called sliding dropouts. Sliding dropouts are separate pieces that are bolted onto the main frame of the bike. The actual dropouts are vertical (like on a geared bike), so you can still use quick release hubs. Then, to adjust the chain tension, you slide the whole wheel and dropout assembly forward or back as needed, and tighten the dropouts in place to the frame. With disc brakes, the brake caliper is attached to the dropout, so the brake moves along with the wheel, solving the disc brake issue.
Singlespeed mountain bike with disc brakes and sliding dropouts
Mountain bike with an eccentric bottom bracket
A typical "push-style" chain tensioner
Another option for converting a bike to a singlespeed is an eccentric rear hub. This operates on a similar principle as the eccentric bottom bracket, but the adjustment is done at the rear wheel rather than the bottom bracket. The axle on the rear hub is positioned offset in relation to the points where it inserts into the frame dropouts, and the amount of offset can be re-adjusted, thus providing a way to tension the chain. This is a much more elegant and durable solution than a chain tensioning device, and is suitable for off-road riding. The downside of this option is the expense; eccentric rear hubs start around $160, and you'll probably need to have your bike mechanic custom-build a wheel around it, so figure on about $25 and up for a rim, $1 for each spoke, and about $50 for labor. Pictured below is the White Industries "Eno" Eccentric Rear Hub:Finally, the big question when it comes to singlespeed riding is, "What gear do I want?" That depends on where you'll be riding. For off-road riding, a typical singlespeed gear combination is a 32-tooth chainring up front, with an 18-tooth cog on your wheel. This gives you a low enough gear to be able to crank up most of the steeper climbs you find on a mountain bike trail. If you tried riding this bike on pavement, though, you'd find that you'd be spinning your feet out, and "going nowhere fast." For road riding, most people find that a 42-tooth chainring and a 15- or 16-tooth cog make a good combination, enabling you to maintain a good, steady pace on flat terrain, with a gear low enough to climb up typical road grades. Single cogs are not very expensive, so once you get the bike set up, it's easy to experiment with different-sized cogs to find the gearing that works best for your needs.