Just the size!
A California King (72 x 84 inches) is four inches narrower (side to side) and four inches longer (head to toe) than a King (76 x 80 inches). The difference simply comes down to the shape. Many sleepers love the extra width that the King provides, especially those with children and/or pets who like to pile into bed on those long, lazy weekend mornings. The California King, in addition to providing prime real estate for your dog who loves the foot of the bed, is a popular choice among taller sleepers.
When buying a mattress to use with an existing bed frame, we strongly recommend double-checking the size of your current mattress (trust us on this one). A few measurements ahead of time will help make sure that you don't end up trying to fit a California King mattress into a King-sized bed frame (even though if it happens, we're happy to help you get it sorted). The same goes for double-checking on sheets.