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, the class will re-open. You can check back on this page or to easily keep track of full classes, you can add this class to your list in the 'Check Class Status' page in MySite. To petition a class, attend the first class meeting and ask the instructor to add you. This will often be successful, as there are usually a few empty seats on opening day.
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.