The category spans studless ice-and-snow models (the mainstream choice), performance winter tires for faster driving on cold, mostly-clear roads, and studdable designs for extreme ice. Tradeoffs: soft winter compounds wear quickly in warm weather, handling feels softer on dry pavement, and you'll need a second set of tires or wheels with a seasonal swap.
Buy winter tires if temperatures sit near or below freezing for weeks at a time where you live — all-wheel drive helps you accelerate, but only tires help you stop. Mount them when temperatures consistently drop below about 45°F and remove them in spring to preserve tread life.