In essence, they are the same, depending on if it is used as a question or in the negative.
Rwyt ti = You are.
Wyt ti = You are?/Are you?
Dwyt ti (ddim) = You are not.
It was also discussed here, where you can find a little more detail: https://www.duolingo.com/comment/13264421
I appreciate the reply. I am confused now because I remember seeing "Dw i ddim yn...." used in previous lessons instead of the negative "Dydw i ddim yn..." which other sites say is correct.