What's the difference between "winter" and "summer" tyres?

Ok I know that obviously the winter ones are more stable and have a special design that makes them more steady on snow, but there must be some disadvantage too, otherwise why use anything else? Are they more expensive? Do they melt in hot weather? Or are they not as fast? I recently had an accident with brand-new winter tyres and think they may have had something to do with it.