Like traditional classes, online classes involve interaction with your professor and with other students. There will be readings, assignments, lectures and learning activities that engage you with the content and with other learners. Some courses have deadlines while others are completed at your own pace.
Once you've selected a specific course, there will be instructions on how to proceed with the course. As an example, how to follow the syllabus, specific materials required for the course, how to contact the instructor with questions, etc.
The course might require papers, case studies and/or exams. There could be discussions, small group activities or simulations, experiments, and assignments that are completed offline.
You'll be guided through the course by the instructor, and assigned specific learning activities.
In order to achieve the most from the course you should take an active role in the learning process, and regularly contribute comments and questions to the discussions.
Most of the courses has set up a system for communication with the instructor for help in completing the course.
Some of the courses offer certificates or credit for completion. Check the individual College or University to see the requirements for the specific course you're taking.