I have been seeing this question on Ravelry a lot lately and think it is like trying to describe America to a foreigner. It is so many different things to so many different people that you can't just get a little sound bite of information and run with it.
When I was waiting for 3 months to join, I imagined that the Ravelry streets were spun in gold and all of my knitting problems might possibly vanish. That Everything I might want or need would be at my fingertips. And when I joined, it was... then I wanted more. I wanted better searches, more detailed information and different ways of organizing.
So I brought them up to Casey and Jess to see if they were worthy of Ravelry, and some were others were not (thoght to tell the truth the ones that didn't make it had good reason not to be accepted... like a better idea already in the works...) and those that were worthy made it on "The List". Casey works on The List everyday and every Raveler can look up the list and see what has been checked off or not and just see what shows up. Some things I didn't even know I wanted show up too, like being able to see what yarns have been sold at a local yarn store and their locations and project recommendations for a yarn I have.
But What IS Ravelry? It matters how you use it.
If you dye yarn, you can add the yarns to the yarn search and be one of the top 12 newest yarns for a bit. You can advertise on there too and make a group for members that like/love your yarn. You can see all the people that own your yarn and how they have used it.
If you write patterns, you can add them to the Ravelry pattern database and be one of the 6 newest patterns for a bit. You can advertise, make a group, and even see what people have done with your patterns. Did they make it something that was supposed to in a bulky yarn in lace weight so as to make it fit a child?
Need a yarn for a pattern you have fallen in love with? Want it to be a silk yarn that you can buy locally? Head on over to the advanced search. If anyone has bought a silk yarn and posted it on Ravelry, that yarn will come up on the search. (this does not include chain craft stores like Hobby Lobby, Michaels, Jo Anns, A. C. Moore or Wal*mart).
Need a pattern for this lovely Soy yarn you have but don't want to have to do math in resizing to make the pattern work for you? Search the weight of the yarn in the pattern section and you will see a bunch.
Don't know how to use the searches? Ask on the yarn or pattern forum and someone will help you. We have all been in need of help before and someone is always willing to help out. (Though not do the work for you... they do expect you to learn...)
There is a bunch more too. You can search yarns based on who wants to trade or sell yarn in their stash, search finished objects, or get advise about how to dye yarn. So much to do on Ravelry, so little time.
I have a strange feeling that someone is going to copy and paste this and try to pass it off as their own... Don't, I hate people that plagiarize so don't even think about it. Link to this post if you like it so much.