What are the differences between these campus programs?

Hillel is basically a Jewish Student Union. It has social events; provides religious services, especially on holidays; schedules speakers and programs on topics of Jewish interest; etc. Some are more political; one on my campus would routinely schedule protests on human rights issues.

MEOR provides Jewish studies for students. This is just the sort of program that a Hillel group might bring to campus, holding Meor studies at the Hillel facilities; on other campuses Meor and Hillel may have separate facilities.