What is the difference between early action, early decision, and rolling decisions?

Early action, as the name implies, is when you act early by applying early. This shows colleges that you have started your college search earlier and may give your application priority in some colleges. Other Universities like MIT don't care.

Early decision, as the name implies, is when you have decided early on your top choice and know where you want to go against all other choices. This assures colleges that you will attend their college if accepted and given decent financial aid. This application is legally binding and requires you to attend the college, so don't apply to two early decisions if you don't have a lawyer (there may be a fine). Many colleges don't offer this, as they are not desperate for students. Not that those who use it aren't good.

Regular decision is when you apply when most other people are applying which is usually early in the year of your graduation. Nothing binding or special about that. Just plain old applicant Joe.