Object-Orientation FAQ

"Parametric polymorphism is obtained when a function works uniformly on a range
of types; these types normally exhibit some common structure.  Ad-hoc
polymorphism is obtained when a function works, or appears to work, on several
different types (which may not exhibit a common structure) and may behave in
unrelated ways for each type."  

Parametric polymorphism is also referred to as "true" polymorphism, whereas
ad-hoc polymorphism isn't (apparent polymorphism).