Red shift (occasionally redshift) is the observed reduction in frequency of electromagnetic radiation perceived when the source of the radiation is moving away from the observer. (It is analogous to the frequency shift heard when sound emitting objects move relative to the auditor (Döppler shift).) A corresponding increase in frequency observed from approaching sources is known as blue shift.
Shift (red and/or blue) is detectable even at speeds commonly encountered in everyday life. Radar and laser speed guns and radar generally use shift measurement of reflected radiation to ascertain speeds of moving objects.
Astronomy uses the frequency shift of various known marker lines in stellar emissions to determine the relative rate of approach (rare but existent) or departure of objects at any distance from the observer. It was the conclusion that distance was proportional to red shift (the further away an object, the greater its speed of departure) that caused Hubble to propose that the universe is expanding.
Redshift quantization (a 'stepped' increase rather than a continuous smooth increase in shift) was first investigated in 1973 although recent redshift studies have produced no supporting evidence beyond explained galaxy clustering.  Creationists and geocentists have taken (manipulated?) the data to indicate that our galaxy is at the "centre" of the universe, thus disproving the Big bang theory and allowing a return to the God created Universe. 
 Interstellar reddening
If a gas or dust cloud lies between an astronomical object and the observer, the light is "reddened" somewhat. This effect is markedly different from a red shift, however. In a red shift, the spectral lines appear at lower frequencies. Interstellar reddening doesn't effect the frequency of the spectral lines, it merely reduces the intensity of higher-frequency light, leaving more lower-frequency ("redder") light to arrive at the detector.
In other words, reddening due to material between the source and the observer does not explain the red shift seen for very distant objects; only motion of the source away from the observer can explain the red shift.