It's an optical effect due to the atmosphere.
Stars twinkle because of turbulence in the atmosphere of the Earth. As the atmosphere churns, the light from the star is refracted in different directions. This causes the star's image to change slightly in brightness and position, hence "twinkle."
In large reflecting telescopes there's also the effect of diffraction by the secondary mirror supports which leads to the characteristic cross shape lines on bright stars.
Stars are point sources (being far away objects) whereas planets are extended sources (because they are close to earth). Now when you look at the sky the star light is diffracted by various atmospheric layers through which starlight is reaching you. Slight fluctuations in density temperature etc within atmospheric layers is natural. This make the starlight fluctuate in intensity as well as in directionality causing it's so called "twinkle". Scientific term for it is "astronomical scintillation"
Planet's also scintillate, however being extended objects these "twinkles" average out for them. Thus with a necked eye planets don't appear to scintillate.
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