Before we go into the answer directly we should know what is doppler effect. Well, have you noticed one thing? When a train whistles pass you how the sound of whistle become shriller when it approaches u, and just after crossing you, the sound become deeper. That is the the doppler effect. It says, if the emitter (source of energy) and receiver are moving towards each other, the frequency seems to increase and when they are moving away from each other the frequency seems to decrease. This is because the emitter is either catching up with or lagging behind each wavefront that it emits, making the waves closer together or farther apart. Same thing happens to stars (or any other celestial objects) emitting light (or other forms of electromagnetic radiation). If the spectrum moves towards blue (higher frequency of VIBGYOR) we say blue shift and if it moves towards red we say it is red shift (lower frequency of VIBGYOR). So, blue shift means relatively travelling toward us and read shift means relatively travelling away from us. But remember, the emitting objects must have enormous speed. And also that, the rules of general relativity holds in these case which has other explanations like time dilation due to gravitation force or expansion of space for these shifts. But let that not bother u as they are some high level topics.

The blue shift means that light is moving towards you and the red shift means that light is moving away. Right now light from distant galaxys is red shift so this means that they are moving away from us and the universe is expanding.That's laymens terms. :)

In 1945 American astronomer Edwin Hubble discovered a analogy between the distance of a galaxy from the Earth and the speed at which the galaxy was moving away from it. He found that the farther the galaxy is from Earth, the faster it is receding. This analogy is so consistent (is until today proven true on any occasion) that is known as Hubble's Law. Hubble proved his theory using a spectrometer & a spectrograph through which he noticed that the Fraunhofer lines (dark lines in the spectrum caused by the absorption of specific electromagnetic radiation wavelengths by atoms of gas in Earth's atmosphere) were shifted towards the red colour in the absorption spectrum. The obvious conclusion was that the absorbed radiation emitting from the galaxy had increased it's wavelength. The lines' shift towards higher wavelength values is called redshift, while a reduced wavelength suggests a blueshift. Redshift was just the tool Hubble needed to conclude that the universe was expanding, which acted as evidence against the Steady-State theory & in favor of the Big Bang theory & the correctness of Einstein's relativity equations. Redshift & blueshift are astronomy's equivalents of Doppler's effect, which states the frequency increase of any emitted light or sound wave relative to an observer, as the wave's source approaches the observer & the wave's frequency decrease as the source moves away from the observer. Astronomers measure the redshifts of quasars to determine their distance from Earth. A redshift near 5, for instance, means that the quasar is about 12 billion light years away from Earth (with 1 light year being equal to 9.5 trillion km).

Some good answers here. I would like to add that the reason it shifts color is to maintain the speed of light. The speed of light is the same for everyone, even if I shine a flashlight on you and you are heading toward me at half the speed of light. The speed of the light that hits you from my flash light is not lightspeed+ half-of-lightspeed. Its just the speed of light. But the light would be severly color shifted to counter-act this, its a way of siphoning off what would have been added energy. It sends that energy into color spectrum frequency energy. http://www.answerbag.com/q_view/84490

Blue shift is the phenomenon that the frequency of an electromagnetic wave (such as light) emitted by a source moving towards the observer is shifted towards the blue side of the electromagnetic spectrum (that is, its wavelength is decreased). The phenomenon of shifting wavelengths in frames of reference moving relatively to each other is commonly known as Doppler shift or Doppler effect. While the general redshift of starlight is seen as evidence for an expanding universe, there are a few examples of blue shift in astronomy: The Andromeda Galaxy is moving towards our own Milky Way Galaxy within the Local Group; thus, when observed from earth, its light is undergoing a blue shift. When observing spiral galaxies, the side spinning towards us will have a slight blue shift . Also, Blazars are known to propel relativistic jets towards us, emitting synchrotron radiation and Bremsstrahlung that appears blue shifted. In physics and astronomy, redshift occurs when the visible light from an object is shifted towards the red end of the electromagnetic spectrum. More generally, redshift is defined as an increase in the wavelength of electromagnetic radiation received by a detector compared with the wavelength emitted by the source. This increase in wavelength corresponds to a decrease in the frequency of the electromagnetic radiation. Conversely, a decrease in wavelength is called blue shift. Any increase in wavelength is called "redshift", even if it occurs in electromagnetic radiation of non-optical wavelengths, such as gamma rays, x-rays and ultraviolet. This nomenclature might be confusing since, at wavelengths longer than red (e.g., infrared, microwaves, and radio waves), redshifts shift the radiation away from the red wavelengths. A redshift can occur when a light source moves away from an observer, corresponding to the Doppler shift that changes the frequency of sound waves. Although observing such redshifts, or complementary blue shifts, has several terrestrial applications (e.g., Doppler radar and radar guns), spectroscopic astrophysics uses Doppler redshifts to determine the movement of distant astronomical objects. This phenomenon was first predicted and observed in the 19th century as scientists began to consider the dynamical implications of the wave-nature of light. Another redshift mechanism is the expansion of the universe, which explains the famous observation that the spectral redshifts of distant galaxies, quasars, and intergalactic gas clouds increase in proportion to their distance from the observer. This mechanism is a key feature of the Big Bang model of physical cosmology. Yet a third type of redshift, the gravitational redshift (also known as the Einstein effect), is a result of the time dilation that occurs near massive objects, according to general relativity. All three of these phenomena, whose wide range of instantiations are the focus of this article, can be understood under the umbrella of frame transformation laws, as described below. There exist numerous other mechanisms with very different physical and mathematical descriptions that can lead to a shift in the frequency of electromagnetic radiation and whose action may occasionally be referred to as a "redshift", including scattering and optical effects (for more see section on physical optics and radiative transfer).

It describes the way in which the wavelengths of light are affected by the directional velocity of stars and galaxies. The light emitted from those moving toward us is in shorter or slightly more compressed wavelengths which registered as the Blue shift, and the light from those moving away from us has longer "red shift" wavelengths. The same thing happens with sound, which we are all aware of. When an ambulance is speeding toward you, the wavelengths of its sound are bunched up and short, so the sound you hear is a higher pitch, but as it goes past you and speeds away, the wavelengths of the sound are now stretched out longer as they travel to your ear, and the sound you hear is a lower pitch. With sound and light this phenomena is called the "Doppler effect" The difference being with light the wavelength changes register as colour whereas with sound, they register as pitch differences.