A derogatory term for a clueless and malicious hacker. Usually used specifically to refer to someone who doesn't really have much in the way of visible skills themselves, but simply looks around for a rootkit and then tries to apply it to everything in sight.
Script kiddies would not be particularly dangerous without black-hats to supply them with interesting things to do. They are, however, becoming more and more of a problem as the years go by and the internet becomes more and more all-pervasive - they magnify the scope of any given security breach by many orders of magnitude, both because of their sheer numbers (it's a lot easier to be a script kiddie than a black-hat) and because script kiddies are a lot more promiscuous in their activities - a black-hat generally won't attack a system unless he has a distinct use for it, but a script kiddie will try to "pwn" anything and everything in sight.
It is generally considered a DIRE insult to refer to someone as a script kiddie.