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. Most of the Kings found throughout large cities were one of the first artists to start tagging and creating pieces in that 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 guarantee 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. Pieces created by a King will most likely be spaced from other artists work, usually as a symbol of respect to the King.