http://en.wikipedia.org/wiki/Redshift
Absorption lines in the optical spectrum of a supercluster of distant galaxies (right), as compared to absorption lines in the optical spectrum of the Sun (left). Arrows indicate redshift. Wavelength increases up towards the red and beyond (frequency decreases). [img]http://upload.wikimedia.org/wikipedia/commons/thumb/1/14/Redshift.png/170px-Redshift.png[/img]


Redshift and blue shift [img]http://upload.wikimedia.org/wikipedia/commons/thumb/e/e4/Redshift_blueshift.svg/220px-Redshift_blueshift.svg.png[/img]