Rollers generally consist of three cylindrical drums, connected via a belt which allows them to rotate beneath the wheels of the bike.
Riding the rollers requires balance – and it can take some time to master the technique. Learning how to do so is often a journey that begins between two door frames (where there's nowhere to fall), with the rider gradually reducing their reliance on the support of a solid surface as confidence grows.
The shape of rollers doesn't change that much between brands - but there are a few variables. Differentiating factors include how compact they are when put away, how easy they are to ride (some have grooves, higher sides or a curved parabolic shape to help you out) and the amount of variability in the resistance on offer.