Ravelry is hugely different than a forum. Ravelry has forums, but they are only a small part of the big picture.
Sure, you can ask people what they're working on, and they can tell you and post pictures. But on Ravelry, YOU click on one link to see EVERYONE who is working on the same project you're working on. You can see what yarns they're using, the needles, any comments they or others have made, etc.
Then lets say you have a couple skeins of yarn and you don't know what to do with it. You can search that yarn on Ravelry and again see all the projects people are knitting with it, along with the links to the patterns they're using, whether it's a free pattern online, or what book they're using and where to get it.
This is just a teeny little explanation. I really recommend you view the Ravelry Video Tour
to get a better idea of what it's all about. There's a reason they already have over 30,000 members and people waiting to join in a little over 4 months!