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.
Classes of a higher level are typically more difficult than classes of a lower level. You will see that your general education courses are typically lower division while major and specialization courses are upper division. Elective courses are usually a mix of lower and upper division. Be sure to check the Academic Catalog to see a full listing of courses.