Overshoes as the name suggests are bootie type covers which the cycling shoe fits into to help keep the elements out and heat in. They have pre-cut holes in the feet for cleats and heel tabs which helps prevent damage when walking in and should make them last longer. Fastening is usually via a zip at the back with a Velcro tab at the top to keep them in place on the calf. They are available in many different weights and styles, from simple sock type shoe covers to thick water and wind resistant types and everything in between. Overshoes are mostly used on road bikes but there are also several suitable for mountain bike shoes and their more aggressive sole treads so check manufacturers recommendations for more details.