A tire that is approved for use in winter is a tire that has been designed for winter use. It has been designed with winter in mind, so that the rubber compound will remain soft even at low winter temperatures. The tread has been optimized for the winter weather conditions that include, snow, slush and ice.
Summer tires have a rubber compound that is vastly different from that of a winter tire. The rubber compound in a summer tire will lose the grip when the temperatures drop because the rubber becomes hard; therefore a rubber compound in a winter tire is completely different.
A winter approved tire has the properties needed during wintertime. The grooves are designed to handle snow and slush. The tread design is very different between summer and winter tires. In winter you can have either non-studded tires or studded tires, which will perform especially well on ice.
If you drive with a winter approved tire, then you are ready for the winter when it arrives. You can drive safely on snowy, icy and slushy roads. It is definitely not safe to drive with tires that are not winter approved when you have harsh winter conditions.