All courses at and above the 400-level qualify for graduate credit. Students must earn at least 18 credits at the 500 level.
Grades of C or higher count towards Certificate Programs. Grades of B or higher count towards the master's degrees. So, for example, a grade of C in GEOG 485 would count towards the GIS Certificate Program, but not the MS SDS Program.
If a student takes a term off he/she will need to fill out the "Resume Study/Change of Degree or Major" form before registering for subsequent courses. This form is available at http://www.gradschool.psu.edu/forms-and-documents/ges-owned-forms-and-documents/reschg2pdf/.
The Graduate Degree Programs Bulletin is available at: http://www.psu.edu/bulletins/whitebook/