Neither of those are German words, but I'm guessing you're asking about Kleider and Kleidung (capitalised).
The difference is about the same as between "clothes" and "clothing" -- i.e., they mean more or less the same, but Kleider (clothes) is grammatically plural while Kleidung (clothing) is grammatically singular.
Further, in German, Kleider can also specifically mean "dresses" as well as generically "clothes".
Like English "clothes", Kleider doesn't have a singular -- you can't talk about "one clothe" or ein Kleid in the sense of "one article of clothing". (ein Kleid can only mean "one dress".)