The difference between the two endings is familiarity. "야" is used between close friends or when you are talking to someone younger than yourself. For example, if your close friends says or does something silly you can say "바보야". "다" is the verb stem ending. It's how the verb looks before you conjugate it into any of the other forms. You use this ending when making statements. So if you wanted to say that guy is silly: "그남자 바보다". At least that's how I've understood it when it's been explained to me. Hope this helps!

If you said to your friend that she/he is a foolish, "너는 바보야" (you are so foolish) If you said that a person is very foolish to your friend, "he/she is very foolish" (걔 진짜 바보다) +++ "~야" is usually used when a person indicate familiar acquaintances (friend, family or the others) So if John and Marry are friends, you can say "John아, Marry야"