Click and Clack, the Tappet Brothers - in real life Tom and Ray Magliozzi of NPR's Car Talk - have this to say about tires: "If you live in an area where it snows a fair amount, you should get four good snow tires. Nothing will make a bigger difference."
Here's the secret about snow tires, or winter tires, or all-season tires: Every manufacturer puts out good ones. The difference in performance is not from choosing Brand A vs. Brand B, it's in making sure the tires are the right size for the car, and the right feel for the driver. Some tires are more aggressive than others, with deeper tread and larger lugs, and offer a noisier ride on dry pavement but more traction in snow. Other tires have less pronounced tread, and thus somewhat less traction, but offer a quieter and smoother ride on dry pavement. The choice is personal, and depends on a driver's tolerance for noise, and the amount of on-snow driving done in a winter.
Buy tires before need for them arises. Discuss options with the tire dealer. Never mix radials and normal tires.
The American Automobile Association advises drivers to check their tires for wear, and to make sure they are properly inflated. Colder temperatures mean a difference in air pressure. The formula is 1 psi for every 10 degrees Fahrenheit of temperature. Winter in snow country can mean an average temperatures 50 degrees colder than summer, or 5 psi in tire pressure, which will affect a tire's performance, and make it wear faster. Drivers should check tire pressures often, before setting out, when the tires are cold, especially in fall and spring when temperatures are changing most dramatically.