Why not just drive with your winter tires in summer? Then you don't have to pay for two sets of tires and for changing them. The reason is simply that it’s dangerous – you're putting your safety, that of your passengers and other drivers at risk. Tires are a major factor in how safe your vehicle is to drive. So what's the difference between summer tires and winter tires then?
The rubber compound
The amount of rubber used in winter tires helps the tire stay soft and flexible so it can grip the road when it's cold outside. If you drive on winter tires in the summer, then your tires will be too soft, which means they will wear faster, reduce fuel efficiency and also need a greater distance for braking. The reason for this is because winter tires are more pliable at higher temperatures, so they wear more quickly on hard, dry asphalt.
The rubber compound used in summer tires is considerably harder than it is in winter tires so they can handle the heat of summer. If you drive in the winter with summer tires, stopping distances will be longer and it will be harder to drive in a straight line because the tires aren't soft enough to grip the road. If you're even able to get going, that is…
Summer tires have large contact patches which give the vehicle a better grip on the road. Although the tread pattern has fewer grooves and sipes (thin slits that cut across the rubber) than that of winter tires, the grooves are bigger so they can move large quantities of water away to the sides, maximizing the contact with the road in order to avoid hydroplaning.
Winter tires, on the other hand, have a lot of grooves, which are also deeper than those in summer tires. It is these grooves that allow winter tires to keep their traction on snow and ice. There are also smaller channels called sipes that help the grooves keep the tire in contact with the surface.
Your vehicle will skid if you drive on summer tires in winter – see for yourself what that looks like and how winter tires fare in the summer.