imagine you have a rope and it has become all tangled up, this is a "ravel", if you sort it out and un-tangle it, you are unraveling it.

That is a great question! "Unravel" is very common in the U.S. - people use this word to mean to fall apart - like when the fabric in a sweater starts to fall apart; "My sweater is unraveling." We can also use this word idiomatically, as in "Their relationship is unraveling." "The agreement between the two countries is starting to unravel." It also means to explain clearly, to illuminate, to reveal. "Let me unravel this mystery for you." We also use this word when talking about an illogical story in a book, movie, etc.. "I liked that movie at first, but then it really began to unravel (to fall apart) in the final part." (That is, the final part of the movie doesn't make any sense.) I'm from the U.S., and I've never heard of anybody using "ravel." I had to look up this word online. It can mean both to to disentagle (same as unravel) and to entagle, both literally and figuratively. So, while "ravel" has opposite meanings, "unravel" is used only in the sense of disentagling or loosening the threads (of a fabric, a mystery, a relationship, or a story.) And, I've only heard people use the word "unravel," never "ravel" in the U.S.

There are various meanings for ravel. Sometimes the same as unravel, sometimes not. It depends on whether ravel is a verb or a noun, with or without an object. See here for more details: https://en.oxforddictionaries.com/definition/ravel