QuickCheck is a library for random testing of program properties.

The programmer provides a specification of the program, in the form of properties which functions should satisfy, and QuickCheck then tests that the properties hold in a large number of randomly generated cases.

Specifications are expressed in Haskell, using combinators defined in the QuickCheck library. QuickCheck provides combinators to define properties, observe the distribution of test data, and define test data generators.

You can find a (slightly out-of-date but useful) manual at http://www.cse.chalmers.se/~rjmh/QuickCheck/manual.html.