What is a 'Teacher'?

I was reading a little bit in 'E-Learning by Design' by William Horton, and came across a question that made me wonder about my role as a teacher. The question was one of a list meant to help the reader decide whether to skip the first chapter. The question reads, in part, "Do you lack either education and experience in instructional design? Perhaps you are a subject-matter expert or instructor..." So which am I, as a teacher? Am I a subject-matter expert? An instructor? An instructional designer? What does it mean to be any of these? Does an instructor have to be a subject-matter expert and/or an instructional designer?

My initial answers to these questions is that each of these are separate things. To be one is not necessarily to be the other. College professors, for example, may be subject-matter experts in their chosen field, but may not be able to instruct others in similar pursuits, and may not be able to design quality instruction. Is there value in becoming a professional with training in all of these areas? If one did obtain such training, would they be able to more effectively help students learn? Should all teachers receive specific training in each of these areas, or should teachers team up based on their strengths and have specific tasks assigned them? Should there be teachers working behind the scenes to create lessons for those who are actually in the classroom?