For most courses, you will not be required to attend class at a set time. This is called "asynchronous"
instruction. You will complete your work on your own schedule each
week, although you will still have to submit assignments and complete
exams on time as requested by your instructor.
Some instructors may have set times for "live" Web discussions. This means that you may need to "log on" to your online course at a specific time, even if most of your work is done on your own schedule. And a few online courses are "synchronous", meaning that you will have to be online at specified times along with the instructor and the rest of the class.
Class descriptions in WebAdvisor and the Course Orientation Letters will clearly state whether a particular course is synchronous or asynchronous.