The Haskell 98 ReportHaskell 98 Report has a section on thisOperator Applications that clears it up:
An operator is either an operator symbol, such as + or $$, or is an ordinary identifier enclosed in grave accents (backquotes), such as `op`. For example, instead of writing the prefix application op x y, one can write the infix application x `op` y. If no fixity declaration is given for `op` then it defaults to highest precedence and left associativity (see Section 4.4.2).
As indicated by the other answers, the Report also has a section on Fixity Declarations that allows you to define your own fixity, for example:
infixl 7 `op`