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Educators (A,B,C,D): Teachers, Instructors, Authors, and Tutors are all educators in the sense they teach, instruct and show the learners online and onsite how to do things and understand the World.
The teacher (A) is the person who teaches and communicates with learners, and learners can communicate with the teacher. It is synchronous communication. Teacher show as Instructors do, but learners can ask teachers questions and get a reply.
Tiger Woods is not a teacher unless he is on the golf course with you or in a webinar answering your questions directly. In a YouTube video on how to play golf, Tiger Wood is an instructor, not a teacher.
In a webinar, a teacher (A) calls the webinar and presents the lesson. If the webinar is saved for later use, the webinar will be a video, and the learners can not communicate with the educator; hence, an instructor (C) now leads the former webinar, which is now a video.
The tutor (B) is a teacher who leads an onsite (or offsite) course and determines the grade. The Tutor can be said to be the Main Teacher of the Onsite or blended Course if there are many teachers (A) or instructors (C) in the course events.
If the online course does not have any events with synchronous moments where you can communicate with a teacher (C), the Course has not a Tutor but an author. The author does not need to be an instructor, just as a person who puts songs in a playlist does not need to play music; He is simply the person who has put the course together.
Why is this distinction between the roles important?
We could have Educators as the only term here. Still, just as we have a role for Headmasters and Teachers in a school, the distinctions are important for understanding who is in charge of the teaching progress and whether there is anyone you can talk to face-to-face during the course.
In a purely online course with only instructors, you are left alone and will have the "Support" to contact if anything is inaccurate or wrong or if you have any questions. You are unlikely to talk to anyone personally on Google, Facebook, or Twitter about how you manage to post content on social media. And you are about to get used to doing the checkout yourself in grocery stores around the globe. Human interaction face-to-face is leaving us.
We feel sorry for this development because knowledge is best transported through stories. Told by a classic teacher in a classroom. But today's reality is different. There are instructors you will love telling more stories and teaching you more that you will never have the chance to chat with. Even if they are still alive, It might be Simon Sinek, Tony Robbins, or anyone famous for having a massive audience on Youtube or TED in any matter. And some great teachings on TED are still available, albeit the person is no more with us: like Hans Rosling.
Excellent teaching is still within reach of these roles.