There are certain roles that we usually use at certain stages of the lesson.
For example, we are planners before the lesson and may be monitors during group work and pair work activities. Sometimes we take on more than one role at the same time.
For example, we might monitor and explain if a pair of learners is having problems with an activities: we monitor to see how well they are doing and we explain to help them do better.
There are various names for the different roles of the teacher. The ones above are very common.