Well, the planet rotates and revolves around its sun. The thing is, the rotation axis and the orbit are not strictly perpendicular, which means each hemisphere points alternatively towards the sun, where it receives light on a smaller surface and during a longer part of the day, and away from the sun, where it receives less light during a shorter time.

[Stats tries to draw a picture showing a planet relatively to its sun during different seasons, with axial tilt, sun rays and all. Picture is crappy because Stats isn't quite used to his fingers yet, but you can still figure what's what.]

Sorry, I can't draw quite well yet.