EA (early action) is non binding. You don't have to go if you get in. You can apply to more than one EA school unless the school is SCEA (single choice early action.
ED (early decision) is binding. You have to go if you get in (which sucks if you don't get good financial aid, so make sure if you do ED, the school promises to give good aid). You can only apply to one ED school.
ED and EA deadlines are usually Nov. 1-15 (depends on school).
RD (regular decision) is non binding. You apply around Jan. 1 -Jan. 15 usually (depends on school).
Another one is rolling admissions. You can apply as early as September and the school fills up spots as soon as they recieve your app. so the sooner you apply the better. This is not the case for the other three. For those you just have to meet a deadline.
Hope that helped. Trust me you will learn so much here on CC. Just don't let it consume your life.
