Rolling admissions is the system in which schools review and make decisions on applications by arrival order, that is "first come, first served" basis.
Most schools use this system, based on my Alzheimer's memory, only UCLA told me they didn't use rolling admissions. Which means they probably (I'm guessing here, I don't really know) waited until AFTER the deadline to decide who's in and who's not.
Most schools will review applications and offer interviews and acceptances according to the application arrival period. Which means people that apply the latest, that is closer to the deadline or after December, are competing for a smaller number of spots (since some of them are already taken). Thus having a bit of aharder time and may have higher chances of getting waitlisted, that if they filed earlier in the period.
But, if someone has a good application that's late, that person shouldn't worry excesively, 'cause they might get in anyway. And if someone else doesn't have good stats and applies early, that doesn't mean anything, they'll still get rejected anyway....
MAYBE one of the reasons BU and NYU have such late deadlines and rolling admissions, is because they both have a very large IDP class. So they can afford the luxury of waiting until February to receive a whole bunch of late applications (after the December part 1 scores are in), that are going to be as good as earlier ones. While the rest of schools (with rather small classes) just take the best from the first bacth.
