As with gloves the overshoe can be used all year round. There are many types available ranging from a toe cover that simply fits over the toe area of your shoe to calf high waterproof Gore-Tex thermal overshoes.
The best sellers are the models to keep your feet toasty, warm and dry in the autumn, winter or spring. They are required especially if you have cycle shoes with vents in as these, although great in summer at keeping your feet cool, let water and cold air straight in and can be impractical in the colder months. Even through the summer months some choose to wear a lycra cover over their well vented shoes.
They are often made of neoprene or Gore-Tex type materials. Like a wetsuit they are designed to stretch over your shoes for a watertight fit. Many overshoes have reflective strips at the back for added road safety.
Time trialists and track riders often wear overshoes (thinner lycra ones) for aerodynamic reasons, as every 100th of a second advantage can be the difference between winning and losing an event.
Most clothing brands have a good offering in the overshoe department. Brands like Endura, Altura, Lizard Skins and even Shimano have a cover for every requirement. Fit is the important thing with these, like a glove they must be a tight fit over the shoe they are covering. Any gaps will allow water and cold air in and once your feet are cold it is very difficult to warm them up again. So better to be too warm than too cold.