Most cars sold in the UK are factory fitted with summer tyres. These tyres are designed to work in all conditions, however they're optimised for warmer temperates. When the temperature drops outside of the optimum zone the rubber hardens up offering reduced grip in both dry and wet conditions.
Winter tyres, also known as cold weather tyres are designed to work in cooler, sub 7c conditions. These are a legal requirement in certain countries and offer far superior handling and grip in winter conditions.
All season tyres are designed to be used year round. They're neither optimum for summer or winter, they do however offer reasonable performance in all conditions without the draw backs of summer tyres in the cold, or winter tyres in warm conditions.