Functional programming languages are designed to treat computation as the evaluation of mathematical functions. They emphasize the use of functions as first-class citizens, meaning functions can be passed as arguments, returned from other functions, and assigned to variables. This approach promotes immutability and avoids changing state, which can lead to more predictable and easier-to-understand code. Common examples of functional programming languages include Haskell, Lisp, and Scala. These languages often support features like higher-order functions, recursion, and lazy evaluation, allowing developers to write concise and expressive code. Functional programming is particularly useful in scenarios requiring parallel processing and complex data transformations.