There are parts of the Bible where the words leap off the page across two or three thousand years to today like we were listening to a Sunday sermon. I experience a good deal of Paul's letters to the Corinthians and Philippians in this way. There are other parts of the Bible where the distance between our world and Bible times is so thick that I'm not sure anyone really understands what was going on. 1 Corinthians 11:10 strikes me this way, where Paul says a woman should have authority on her head because of the angels... what does that mean?
Much of Jude, as we have seen, jumps straight off the page, like the fact that God can keep us from stumbling and make sure we appear before him with great joy (Jude 24). But there are also some verses in Jude where I'm not sure anyone really knows what was going on, and we feel the distance between our world and the world in which Jude lived. It's at times like these that we remember that God revealed the Bible first to the audiences of the Bible, and he did so first in their categories with language and imagery that was part of their world.
So Jude mentions a story straight out of apocalyptic Jewish literature that isn't in the Old Testament and the writing it seems to come from hasn't even survived. As we will see with the book of Revelation, "apocalyptic" literature refers to Jewish literature that dealt especially with revelations about what was going on in the heavenly realm. We'll give a more detailed definition in the next chapter.
The "Assumption of Moses" is a work that some ancient Christians mentioned as they were interpreting Jude's statement that the archangel Michael argued with the Devil over the body of Moses (9). By chance, only one partial copy of this document has survived from the ancient world, and the part of the book that had this story in it hasn't survived. Perhaps it will yet be dug up in some ruin or found in some cave.
This Jewish writing was perhaps from around the same time as Jesus was born. As with many Jewish apocalypses, it talks about current events through the voice of some authority figure from the past, in this case Moses. But you can tell when it was written by where Moses stops predicting the future. So in the Assumption of Moses, the author talks about events up to about the time Herod the Great died. 
Apparently, in the lost part of this writing, the archangel Michael and the Devil argued in some way over the body of Moses.  Jude's point is that the immoral people he is talking about think they can completely disrespect angelic beings--even evil ones--when even archangels don't do that. Even Michael asked the Lord to rebuke Satan. He didn't do it himself...
... continues to talk about the quote of 1 Enoch in Jude 14-15
 This writing is also sometimes called the "Ascension of Moses" and the "Testament of Moses."
 An archangel is a chief angel, of higher rank than other angels. Michael is mentioned as a chief angel in Daniel (e.g., 12:1) and in Revelation (e.g., 12:7). In the prophet Zechariah (3:1-2), it is the angel of the Lord who speaks to Satan and has the Lord rebuke him. No doubt the Assumption of Moses was merging together some of these traditions together.