It's as simple as that. A typical mini course takes 3 - 4 hours to complete. You get all the facilities normally available to students taking the full classes - for three weeks from the time of enrolment. So you have your own virtual desk, class notes and study material, assignments, e-mail contact with your tutor, class bulletin board, and so on, for three weeks. Remember though, if you took an evening class, you'd need to plan in extra time and cost for travel.
Most students enrol for one or two courses at a time.
You're welcome to enrol in as many courses as you like, but remember that you need to be able to commit 2 to 3 hours a week to each course. Allow extra time for any queries or bulletin board communications, and for extra e-mail or discussion time if you have a study partner or belong to a study group. If you haven't chosen your courses yet, check out the Classes page.