A good trainer or set of rollers isn’t just for the when the weather forces us indoors—indoor riding can be great for the time-crunched cyclist, urban riders with limited route options, or anyone recovering from an injury. Even the most weather-hardened winter riders sometimes like to control the environment they're riding in.
Rollers, on the other hand, tend to require a little more concentration. Rollers consist of a set of cylinders attached to an external frame, and work like a treadmill for your bike—you simply get on and start riding. Because you're free to move back and forth, and depend on your own balance to remain upright, it's easier to have a mishap if you're not paying attention. But rollers generally offer the most realistic-feeling ride.