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 are harder the higher in level you go. 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.