[Haskell-beginners] How to interpret the definition of MonadState

[djp at arqux.com]

I am confused about the definition of MonadState. The opening line  
looks like this:

class Monad m => MonadState s m | m -> s where

It is the vertical bar | that confuses me. What does that mean? How  
would one read this line in "natural language", say English?

Thanks.

- J -