In computer science, functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data. It emphasizes the application of functions, in contrast with the imperative programming style that emphasizes changes in state.
Functional languages include APL, Erlang, Haskell, Lisp, ML, F# and Scheme.
Functional programming languages, especially purely functional ones, have largely been emphasized in academia rather than in commercial software development. However, notable functional programming languages used in industry and commercial applications include Erlang (concurrent applications), R (statistics), Mathematica (symbolic math), ML, J and K (financial analysis), and domain-specific programming languages like XSLT.
The lambda calculus provides the model for functional programming. Modern functional languages can be viewed as embellishments to the lambda calculus.
Reference:
http://en.wikipedia.org/wiki/Functional_programming
