Why does the northern hemisphere have a hotter summer and a colder winter than the southern hemisphere?

The tilt of the Earth's axis.In the Summer, the northern hemisphere is tilted towards the sun, therefore it is hotter because the sun's rays hit the earth's surface more directly. 

In the winter, the northern hemisphere is titled away from the Sun, which means that the sun's rays hit the Earth's surface in a slanted manner. Temperature is lower because of this.