[Haskell-cafe] NewbieQ: colon prefix for operators, e.g., Ratio ?

Apologies if this is the wrong mailing list in which to pester folks with Newbie Questions, but I couldn't find my answer after half an hour of scouring the Haskell Report, the Haskell Wiki, the School of Expression book and a couple of tutorials. The frustrating thing is that I know I saw the answer in ONE of those sources and it didn't stick at the time: what does it mean when an operator is prefixed by a colon? For instance, in the Ratio module, the meaning of x % y is clear, but x :% y appears in multiple places and I'm confused.