All season tires are good for most conditions, but when it starts to get icy and snowy on the road, then you are going to want to change over to winter tires. Winter tires have two main differences from normal all-season tires.
First of all, they have deeper treads and extra cuts in the surface. This allows them to gain more traction in wet and slushy or icy conditions than a different kind of tire. The second is that they are made with a different mix of rubber that is a better fit for cold weather. Ordinary tire rubber can get more brittle and change character in the cold, so winter rubber is the best way to keep your performance as good as it needs to be when things get tough.