To be kidding is a colloquialism, technically "kidding" means giving birth to a baby goat, a "kid." However, virtual no one uses it in this sense. For all intents and purposes, "joking" and "kidding" are synonyms. I believe different cultures use one more than the other (in Canada, "kidding" seems more common; in South Africa, where in general the langauge is fairly close to UK English, "joking" seemed more common. That's my experience). They mean the same thing.