Zwift is based around a number of different worlds. Watopia is the largest, and includes flat and hilly courses, all the way up to Alpe du Zwift, a reproduction of the famous Alpe d'Huez climb in the French Alps. The other worlds are mostly real locations – London, Innsbruck, Richmond, Bologna and Yorkshire – although there's also Crit City, which is a dedicated criterium course for races.
There are many different ways to use Zwift. You can simply ride around, or you can join a calendar event which include group rides, workouts and races. You can also opt to follow a training plan with sessions designed to make you fitter. Using the Zwift Companion app you can organise meetups with your friends, and club functionality is also being rolled out.
Zwift currently costs £12.99 a month with no contract.