What are summer tyres, and can they be used all year?

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In places where you get snow it is a good idea to use different tyres/tires.  Not only is the tread pattern different but the rubber itself is stickier on winter tires (but they don't last as long).  If you don't get any snow in your area then you can drive all year on summer tires.  

P.S. - I'm Canadian so I spell "tires" differently but it's the same idea.  Where I live, using winter tires in winter and summer tires in summer is really important, but maybe not so much for Oz.

Typically in the UK which has both summer and a strong winter we tend to buy summer tyres because they can be used all year but are typically cheaper than all purpose. Winter ones don't seem so common