In terms of interview itself, it is the same as off-campus. Just that the interviewers are flying in to the campus or the town near the campus, instead of interviewees going to the company offices.
In terms of process, on-campus recruiting follows a tighter schedule and you will go through the process faster. For example, every year during recruiting season, BCG SoutheEast Asia would fly a team (HR+Principals+Parnters) to the US for campus recruiting. The team would stop by 5-6 cities in a span of 2-3 weeks (Principals might finish by week 2 and fly back after 1st round of interview, while Partners might only fly in later for 2nd round). The company would try to get the process done in 3 weeks or so, since the company cannot afford to have those senior leaders away from case work for too long.
While for off-campus, the timeline is not as strict, and it depends a lot of interviewer's availability. So it might take longer to go through the process.
Hope this clarifies.