CANON

A canon is a type of music that uses an echo of the primary melody by some other voice or instrument at a set interval. The most elemental type of canon is one done in rounds. For example, one group starts Row, row, row your boat... and then another starts Row, row, row your boat... Several other types of canon exist, some very complicated, but all employ this basic musical trick.

One of the most famous, is the Canon in D by Johann Parchelbel (the ch is pronounced as a k) that you all have heard. It was written in the 17th century, during the Baroque period, and has become a standard at wedding ceremonies since the 1970's. Its truly a beautiful piece with many arrangements available. Parchelbel was originally an organist and the piece was written for organ. Here is a sample with strings.
http://www.youtube.com/watch?v=DZHw9uyj81g