What's the difference between joking and joking?

Kidding is a derivative of the verb kid, which means "cajoling", while joke focuses on "mischief"

For example:

I'm not kidding, Frank. There is a cow outside, just standing there. I'm not kidding, Frank. There is a cow standing nearby outside. (equal to I didn't lie to you)

Are you sure you are not kidding me? Are you sure you're not kidding? )