|MadSci Network: Astronomy|
Blue and Red shifts are just variations on a common phenomenon called "Doppler Shift". Any wave (be it light or sound) that is approaching you from a source moving relative to you will appear to you to be shifted in wavelength.
In the case of sound waves, you can easily hear this phenomenon with the sounds of passing cars. As the cars approach, the sound waves produced by their engines are "scrunched up", that is the sound wave is Doppler shifted to a shorted wavelength, and therefore the car engine will sound with a higher pitch that if it was just standing still next to you. The opposite is heard when car moves away from you, the engine's sound will be heard by you to be lower pitched, as the sound waves are Doppler shifted to longer wavelengths.
In the case of light, the Doppler effect is harder to see because the speed of light is so much greater than the speed of sound. So while changes in wavelength are known to correspond to color (from short to long wavelength, the colors of the rainbow are violet, indigo, blue, green, yellow, orange, and red), the change in color due to a Doppler shift is usually imperceptible to the human eye. Therefore, the only easy way to observe this phenomenon is using spectrometers which allow you to carefully measure the wavelength of light you observe. And when a light source approachs you, you can observe in a spectrometer that the light it emits appears to be of a shorter wavelength than when the source is at rest relative to you, that is, the color of the light is shifted blueward of normal. This is the definition of blue shift. Similarly, when a light source is moving away from you, it's light is said to be redshifted, since it is of a longer wavelength, or "redder", than normal.
One of the most interesting discoveries of the 20th Century was made by Edwin Hubble when he looked at the light of distant galaxies and noticed that in every case the light was redshifted, implying that all these distant galaxies are moving away from us. [Moderator's note: the redshifting of galaxy light is what tells us that the universe is expanding. Search our site for more details.]
Try the links in the MadSci Library for more information on Astronomy.