They are constructed from a softer rubber to grip the road surface better in cold conditions. The tread pattern tends to be a bit chunkier and deeper than summer tyres, to accommodate water and clear snow more efficiently. The tread blocks have narrow slits called sipes designed to aid the clearing of water. These features are fully explained on the Which website - Which- winter tyres.
Of course they are! What price your or someone else’s life. Also, when you have your winter tyres on you are not wearing out your summer ones. Just remember to swop them back in the spring.