A single speed has just one speed as you've guessed. However, it doesn't really specify how the rear sprocket is connected to the hub.

There would be basically 3 types:

• Freewheeling. You can backpedal. The bike requires both front and rear brakes.
• Coaster Brake. Backpedal half a revolution then uses a type of friction brake in the hub. Often found on kid's bikes and beach cruisers.
• Fixie. This is the simplest type. The rear sprocket is attached directly to the hub. However, it should have a lockring to hold it in place. The cranks turn when the bike moves, no coasting. One can stop by backpedalling, or skidding. They should have a front brake.