Each online course varies in length. They can be 4 weeks long or 12 weeks long. Some schools call them weeks, units, modules, or lessons. More times than not, theses sections are a week long. I do know of one school that has two-week long units, which can get a bit confusing for students and new faculty. No matter what these are called, the tabs are broken up into sections to help students navigate the classroom. Ideally, they are broken up by week. This allows the student to click on the week they are currently working on and get all of the information they need for that section. Traditionally, these sections will include: lecture, discussions, additional learning resources, quizzes, assignments, and other important information for that section.