The Haskell 98 Report has a section on Operator 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`