Johann Pachelbel (1653 - 1706) wrote his Canon in D major in the late 17th century, then it disappeared for a good 300 years. It didn't mount a comeback until Arthur Fiedler first recorded the Canon in 1940, and until the Jean-François Paillard Chamber Orchestra popularized the piece with two famous recordings (listen here). Now the Canon is everywhere. You hear it at weddings, of course. It finds its way into Beatles' tracks. It goes fantastically viral on YouTube. (This legendary clip has 91 million views.) And now it gets cranked out of music boxes.
This video arranged by Vi Hart has a nice way of stripping things down and reminding us what a canon is fundamentally about. Along very similar lines, you will want to check out this clip showing how a Bach Canon Works. It's pretty amazing.