minBound and maxBound are typeclass functions. Typeclass's are a similar to java interfaces.

So the Bounded typeclass is defined as

    class Bounded a where
        minBound :: a
        maxBound :: a

If you have a function and you want the argument to be Bounded you write

    allLessThan :: (Enum a, Bounded a)=> a -> [a]
    allLessThan x = [minBound..x]

Here `a` is a generic type. That implements the Enum and Bounded typeclass. You could just write `allLessThan x = [minBound..x]` and Haskell with infer the type classes by itself.

To implement the typeclass you use `instance`, like so:

    data MyType = A | B | C | D
    instance Bounded MyType where
        minBound = A
        maxBound = D