Credible comes from latin and means "believable", so "incredible" and "unbelievable" literarily mean exactly the same thing. They are synonyms, but as in all languages the daily use of words might differ slightly - languages are always evolving. I´m not a native english speaker, so I´m not sure about the nuances between these two words, but from my experience I would guess that "incredible" is more often used in a positive sense, while "unbelievable" may have a negative connotation. For example: "That´s incredible! I´ve never seen anyone running that fast before!" "That´s unbelievable! He cheated on you? What a bastard!" These are only nuances and often the two words are totally interchangeable.

In a formal way both adjectives are almost synonyms as mentioned above. 'Incredible' means beyond belief or understanding, unbelievable,astonishing."Unbelievable" has the same meaning as well. "The acting was good, although the plot was unbelievable." "at incredible speed." In fact both of them informally do have both negative and positive connotations. "Incredible" could express disappointment or anger as well as when finding something great ,extremely good ,amazing and marvellous. "Unbelievable" is also used informally to emphasize that something is good and impressive or bad. "He played an unbelievable match." ( extremely good) "They always have incredible stories to tell." ( surprising) " an incredible amount of money." ( extremely great) "It is incredible ,how could she cheat on him!." ( expressing disappointment or anger.) To sum it up ,both of them informally or as slang could mean positively : wonderful,fantastic, superb, smashing, great, fabulous , extremely good and all the emphasized good positive connotations. In a negative sense they could both mean: bad, terrible, awful, disappointing..etc

no difference they are synonyms

It really depends on what the context is. Unbelievable is something that isn't possible. Incredible is something that can be "great", "extraordinary", "surprising"