The red color in the sky at sunset (and sunrise) is due to an effect called Rayleigh scattering. There is a similar form of scattering called Mie scattering, which is responsible for the white color of clouds.
Particles in our atmosphere that are approximately the same size as the wavelength of visible light cause the white light from the sun to scatter and split into individual components. Oxygen and Nitrogen (the main components of our atmosphere) scatter violet and blue light due to their small size. This is why the sky appears to be blue in the daytime, especially at mid-day when the Sun is closest to us.
During sunrise and sunset the distance that the light has to travel from the Sun to an observer is at its greatest. This means the a large amount of blue and violet light has been scattered so the light that is received by an observer is mostly of a longer wavelength and therefore appears to be red.
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