Documentaries are educational and often used in schools to teach various principles.
The two main types of documentary are presentation and narration.
Presentation is where the person is talking to the camera and is continually throughout the video.
Narration is to have a dedicated narrator read a script which is dubbed onto the video, usually at a later stage. The narrator never appears on camera and may not necessarily have knowledge of the subject matter or involvement in the writing of the script.