A course is the basic learning area on Moodle where a teacher displays Materials, Nonturnitin/ Turnitin Assignment, Lecture Capture, Quizs etc for their students
A Resource is any type of static information that you add to your Moodle course—for example, a web page, a link or a text or image file.
Usually an activity is something that a student will do that interacts with other students and or the teacher. Moodle has over a dozen activity types of tools for a teacher to use in a course. All forms of assessment within Moodle are "activities".
The forum module is an activity where students and teachers can exchange ideas by posting comments.