For most of us, all-weather radial tires are sufficient to keep our tires gripping the road as we drive. But if you live in mountainous areas that get a lot of snow, you might need to buy snow tires or snow chains for your tires. Snow tires are made of softer rubber and have smaller treads to allow for better traction. If you decide that you need them, you'll have to switch back to regular tires when the weather changes.
Snow chains are just what they sound like -- bundles of metal chain links designed to fit snugly around your tires. There are also alternatives, such as traction cables, that are made out of cable instead of metal. Before considering whether you need them, look up your state's laws. Some states require them at certain times of year, in certain conditions or when signs call for them. Other states allow but don't require snow chains, and some states don't allow them in certain locations because the chains can damage road surfaces.
Typically snow chains come in pairs because they're meant to be used on the car's drive tires. If your car has front-wheel drive, you'll need chains for just your front tires. Make sure that you get the right type and size for your car's tires. Practice installing the chains before you need them (unless you intend to have a mechanic install them). Snow chains aren't meant to be used on dry surfaces, and driving with them reduces your car's fuel efficiency. You'll also need to drive slower -- generally not faster than 30 mph (48 kph).