They're the same. 'Kidding' is a familiar term for 'joking'. There are various expressions using 'kidding' : 'You're not kidding ?' - 'you're not joking ?' 'Just kidding !' - 'just joking !' 'Kidding' is only used in speaking casually with someone that you know well.

'Kidding' is more informal than 'joking'. It also sometimes involves teasing another person.