The term "King" within the Graffiti universe is used only when reffering to the most well-known artist in a metropolitan area. They are the best of the best, the most respected of the artists in a given area. Pieces made by Kings are typically held in high regards by all the other artists, and to have a piece recognized by a King is a huge honor. Contrary to this, copying or defacing a Kings artwork will almost guarentee that you are cast from the artist society of that area, and will decrease any rep you might have.
The tag of a King can usually be recognized by a crown enclosed above the tag itself, often in quotation marks.