Simple type. AKA nullary type constructor or proper type. Int, String, Double, ... * -> * Unary type constructor. List, Option, Future, ... * -> * -> * Binary type constructor. Either, Function1, Map, ... (* -> *) -> * Higher-order type operator, higher-kinded type for friends. Foo[F[_]], Bar[G[_]], Functor[F[_]], Monad[M[_]], ...