[Haskell-beginners] Help needed in understanding MonadWriter syntax.

Hi, I'm trying to understand the MonadWriter class definition syntax. Now, I'm using GHC 7.6 and the class is defined as class (Monoid w, Monad m) => MonadWriter w m | m -> w where .... I've never seen '|' in the middle of a class definition, could some one explain this? It's used in the guard syntax as well as to separate data constructors in a type definition. Regards Vinay