There are two main types of indoor trainers, or 'rollers'. Only the rear wheel is clamped to a stand at the axle with 'fixed' models, with the front wheel remaining stationary on the ground. With 'loose' rollers, the whole bike floats between three rollers, and the two rollers on which the rear wheel rests also drive the front wheel. This creates a natural ride feel, and you also have to balance. Both systems can be stowed away relatively easily when not needed, and you can usually use your favourite bike with either.