At the undergraduate level, 100 and 200 level courses are considered lower division and 300 and 400 level courses are considered upper division. 500 level courses are reserved for students in a graduate degree.
Theoretically, classes of a higher level are more difficult than classes of a lower level. You will see that your general education courses are typically lower division and major and specialization courses are upper division. Elective courses are usually a mix of lower and upper division. Be sure to check in with the school catalog to see a full listing of courses. You can also view a sample syllabus for each course via the booklist.