What characterizes summer conditions? First of all we are on the plus side of the Celsius temperature scale and it can go quite high up depending on the country in question. Blazing sun on black tarmac can create very hot conditions for the tyres to perform in. The car speed also tends to be higher in summer than in winter, so they also need to be optimised for high speeds used on high performance cars.
With this in mind, the tyres need to cater to high speed, summer weather conditions and of course be durable for wear and tear during these conditions. You also want them to perform well, when the sunny weather turns to heavy summer rain. This is why they have been optimised to prevent hydroplaning with a design that is able to push away the water on the road. Believe me, hydroplaning is definitely something you want to avoid when driving on rainy or wet roads.
So there is actually a combination of things that make a summer tyre great. If you tend to drive at high speeds, you definitely need to look at high performance tyres. If your driving is not so extreme, you still want to get the great handling on summer roads and make sure that you avoid aquaplaning. Make sure that your tyre that has low roll resistance to ensure that you are not wasting money on expensive gasoline, due to bad fuel economy.