Single-speed (SS) bikes are a niche that has a bit of a cult feel to it.  Single-speeders have the reputation of being very tough ("bad-ass") riders but the reality is, single-speeding is not quite as difficult as people think.  SS bikes are incredibly efficient and fast.  Yes, there is a bit of a learning curve at first!  Beginning single-speeders feel like they are never, ever in the right gear - but soon, the rider figures out techniques for riding  that eliminate the need - or desire - for gears, in just about any terrain.  SS bikes are very quiet, there are no expensive derailleurs to break - and best of all, they are "little-kid fun".  The most popular type of SS bike has 29" wheels which are much faster and smoother-rolling than 26" wheels.

A single speed chainring is the same as a chainring on a geared bike, typically a 32-tooth.  The rear cog for most riders will range from a 16 tooth to a 22 tooth.  The smaller the cog, the higher the gearing.  The way to choose the gearing is to ride some favorite trails, notice which gear you're in most of the time (notice that most people have two or three favorites), and gear the single-speed about the same:  for example, if you ride a particular trail mostly in the middle chainring and somewhere in the middle of the cassette, have the single-speed set up with the same gear ratio.  This will give a gear that's useable on MOST terrain:  the two places where SS bikes don't excel at is very steep climbs (which will hurt!) and the flats.

Single-speed riding makes a rider very, very strong.  Most climbing on steep trails is done out of the saddle (for efficiency and because the gearing is typically meant for moderate climbs).  Descending is purely freewheeling - no putting it in high gears and pedaling - this is the time to recover from the climb!  The rider learns to read terrain better, and approach climbs and technical obstacles with just the right amount of speed.  Riding a SS is very basic, very simple, and very fun; and it is great for improving strength, skills and speed (many SS riders can out-ride their friends on geared bikes!).

Any geared bike can be converted to a SS bike.  This is very inexpensive, and can be a temporary conversion - ask your local mechanic!