Comment is a way of writing personal notes to yourself in Netforum activity. Comments are usually visible only to their writer.
OBS. There are few special cases where comments are visible to everybody. These cases are:
User who writes the comment has writen the original message that she or he is commenting. In this case the comment is visible to everybody so that the user can comment his original message without having to alter it.
User is a teacher. Comments written by the teacher are visible to all so that teacher can comment the message in a way that is visible next to the original message.