External Narrator and Focalizer
Having an external narrator and focalizer means that neither the person describing the story nor the character whose point of view we are reading or listening to are part of the story itself. They are observers who have cannot affect the story in any way, such as historians describing a past event. On this narrative perspective, the narrator is emotionally distant, reliable (since he already knows the characters' emotions or the story's events), and omniscient (being aware of facts beyond the knowledge of the characters). This narrative perspective always uses the third person.
External Narrator and Internal Focalizer
On this narrative perspective, the narrator is external but the focalizer is part of the story. The narrator refers to the focalizer in third person ("John saw the man in the blue shirt"), but describes only experiences and actions of the character. It is like the character's personal recollections in third person. The narrator continues to be emotionally distant and reliable, but his perspective has become limited to what the character (John, on this occasion) is aware of.
Internal Narrator and Focalizer
When both the narrator and the focalizer are internal, then the story becomes a personal recollection of events and emotions. The first person is commonly used in this narrative perspective, giving the audience the story through the eyes of the protagonist. The narrator in this case (who is also the hero) is emotionally involved in the story, unreliable, (as he cannot know all the facts an external narrator can), and with a limited perspective.
A rare point of view narrators utilize is the second person. It is a variation of the internal narrator and focalizer perspective, only this time the narrator or focalizer does not use first person, but second person, as if he was talking to himself. For example, a narrator can say, "You woke up and felt dizzy. You drank some water and read the newspaper, before realizing you are in a stranger's house." The narrator remains emotionally involved, while his perspective is still limited to the character's awareness.