There are many different people called Jude in the New Testament, but the most likely candidate for the author of this last of the Biblical letters is Jude the brother of Jesus. While he was probably not a disciple during the ministry of Jesus, he subsequently accepted the gospel message and wrote this letter to warn believers of the importance of fighting for the true gospel and avoiding the moral decadance and lethargy that others were teaching. By quoting examples of the judgments and standards of God in previous eras, Jude shows that the way one behaves and the things one believes are critically important in God's sight.
Q: Is there an archangel called Michael, and how would Jude know about his conversation with Satan?
A: Here is the answer to question one.
Q: How does Jude know what Enoch prophesied when his words are not part of the Hebrew Scriptures?
A: Here is the answer to question two.