Stars twinkle because of turbulence in the Earth's atmosphere. You can think as the atmosphere being made up of several "layers." Each layer has a different temperature and density. As the light from a star passes through the atmosphere, it is bent by each layer, and we perceive the twinkling.
The bending of the light when it passes from one medium to another, like water to air, or one layer of air to another, is called refraction. Refraction is what makes a straw look bent when you put it in a glass of water. Refraction also makes the stars and the Sun near the horizon look higher in the sky than they really are. In fact, when the Sun is setting, we are seeing the sun's disk when the Sun is already below the horizon
You will notice that stars closer to the horizon twinkle more; this is because there is a lot more atmosphere between you and a star near the horizon than between you and a star in the zenith (the point directly overhead) . You will also notice that planets do not twinkle. Stars are so far away that they appear as points of light, but planets are much closer and their disks can seen through telescopes. The fluctuations in the atmosphere are not large enough to affect the light coming from the planets, the Moon or the Sun.
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