A Canon in a church is an official. He may or may not be a priest. The Chapter of a cathedral is an official group made up of the Dean and Chapter of canons, who meet in a chapterhouse. They run the cathedral, especially the business affairs of the cathedral. They are the body which has legal responsibility for the cathedral, and for electing the (arch)bishop.
Originally, a canon was a cleric who lived in the precinct of the cathedral. The word meant a priest who was under (ruled by) a bishop.
In the Anglican church, some canons are laypersons (not ordained). The title is also given to some senior or retired priests as an honorary title.