`Maybe a` is a type:

For a start it's a discriminated union (a sum type), its set of values is:

    a + 1

`a` is the values in the `Just` case, and 1 is the `Nothing` case. It's obvious why they're called sum-types when you view it like this.

`Maybe a` is a type-lambda abstraction (still a type)

`Maybe Int` is a type-application, passing Int to the `Maybe a` type-lambda as an argument, to get a `Maybe Int` in return (still a type).