Type classes are an extension of the traditional Hindley-Milner polymorphic type system. They provide a restricted form of AdHocPolymorphism and allow to introduce overloaded functions into the HaskellLanguage.

They were introduced in the paper "How to make ad-hoc polymorphism less ad hoc" (1988, see http://citeseer.nj.nec.com/wadler88how.html) by PhilipWadler and StephenBlott.

See http://www.haskell.org/onlinereport/decls.html#sect4.1
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See HindleyMilnerTypeInference
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CategoryLanguageFeature