100 - entry level intro, intended for underclassmen with no prerequisites
200 - lower level, usually underclassmen, might have a 100 level prereq or be more subject-specific than a 100 level intro
300 - upper level, usually upperclassmen, usually has low level (100) prereqs.
400 - upper level, usually upperclassmen within that major, often has a few prereqs including 300 levels and is often the highest level for undergrad (will often have "Advanced" in the name)
500 - graduate classes, senior thesis research credits