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Abell 2218 is a large cluster of galaxies. This cluster is so massive and dense that the gravitational field bends the light to produce the pattern of arcs seen in the image. This bending distorts celestial objects that lie on the other side of the cluster. Such distortion acts much like a magnifying glass, focusing these distant objects so that they appear closer and are visible from Earth, allowing astronomers to study galaxies that would otherwise be impossible to see. The arcs are important because they help unravel the mystery of star formation and evolution of distant galaxies. The gravitational force in Abell 2218 is so strong it produces an effect known as multiple imaging, where distortion actually produces more than one image of the same galaxy. This feature has been used by scientists to make a map of the distribution of matter at the core of the cluster, and from these estimates scientists can calculate distance to various different arcs. Abell 2218 is about 3 billion light years away in the constellation Draco. This image was taken with the Hubble Space Telescope.
References:
http://oposite.stsci.edu/pubinfo/pr/95/14.html
http://www.seds.org/hst/A2218.html
http://antwrp.gsfc.nasa.gov/apod/ap980111.html

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