Open classes have empty seats and assuming you meet any other enrollment restrictions, you should be able to enroll in this class.
Important: Beginning the first class meeting day, you must obtain instructor permission to add a class, even if there are empty seats. Your best approach would be to attend the first class meeting, or contact the instructor via email or phone.
Full classes have no empty seats, but if a student drops and the class has no waitlist, the class will re-open. If the class has a waitlist, it will not re-open unless all students on the waitlist get enrolled in the class. To add yourself to the waitlist, go into MySite registration and enter the ticket number for this class.
After the first week or two of class meetings, all classes will close and you will not be able to enroll in a closed class this semester.