The California King is both narrower and longer than its "plain" King size sibling.  Basically this is a good choice only if you're very tall (over  6' 6") but really most folks will prefer the wider King model.  The California King is still very wide - a full foot wider than a Queen in fact - but is 4 inches narrower than a regular King size mattress.

See our size chart for a visual representation of the major mattress sizes.