Haskell is a general purpose, purely functional programming language. This portal points to places where you can go if you want to learn Haskell.
The Introduction on the Haskell homepage tells you that Haskell gives you: substantially increased programmer productivity; shorter, clearer, and more maintainable code; fewer errors; higher reliability; a smaller "semantic gap" between the programmer and the language; shorter lead times.
There is an old -- but still relevant -- paper about Why Functional Programming Matters by John Hughes. More recently Sebastian Sylvan wrote an article about [Why Haskell Matters]. And there is a table comparing Haskell to other functional languages. Many questions about functional programming are answered by the comp.lang.functional FAQ.