The course is organised week by week, with a clear start date and a finish date.
Each week consists of activities. Some of them, like assignments, may have
"open windows" of, say, two weeks after which they become unavailable.
Topics format
Very similar to the weekly format, except that each "week" is called a topic.
A "topic" is not restricted to any time limit. You don't need to specify any
dates.
Social format
This format is oriented around one main forum, the Social forum, which appears
listed on the main page. It is useful for situations that are more freeform.
They may not even be courses. For example, it could be used as a departmental
notice board.