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How it works...

One can look at Monad as a logical extension of Applicative, but with stronger implications. The monad type class is defined as follows:

    class Applicative m => Monad (m :: * -> *) where     (>>=) :: m a -> (a -> m b) -> m b     (>>) :: m a -> m b -> m b     return :: a -> m a     fail :: String -> m a     {-# MINIMAL (>>=) #-}

To define the monad instance, one needs to define the binding function (>>=). In fact, the return function is equivalent to the Applicative pure function. 

The binding function is interesting, with the following signature:

    (>>=) :: m a -> (a -> m b) -> m b

To interpret the preceding function in the context of Maybe, consider the following illustration:

The return function that is equivalent to pure takes in a value and ...