1) Invitation only. Such course is searchable in Google, but instructor has full control on who gets in. This is usually used by school teachers.
2) Open course - course is searchable in Google and anyone in the world can enrol. Such mode usually used by universities what want to promote their courses
First 2 modes are absolutely free for anyone to use. There are no limitation on about of students, courses or content.
3) Open course with paid enrolment. Students can discover and purchase course via a credit card of PayPal, 70% of such revenue its transferred to instructor's PayPal account on monthly basis.
4) Private course. This mode is usually used by private schools, training houses and small companies. In other words institutions who would like to deliver courses only to a limited number of people privately. This mode requires premium subscription. In addition you get extra tools for mass enrolment management, analytics, webinars and increased storage for private videos.