There are various decision plans available for students when applying to college. Not every school offers every program, and you should research your options for each school. I will go over the basic pros and cons of each program, but the ultimate choice has to be yours.
You can update your application with the committee no matter what program you choose. You simply mail in additional information as it happens, and they will attach it to your file. It is best to have everything ready when you apply, but it is acceptable to send it in if you think it will help.
Early Admissions is only offered at some colleges and is for high school juniors who are ready to apply. It requires that the student have fulfilled all college applicant requirements (including SAT exams and college essay). It would be best to wait for senior year, as your choice of colleges are limited and you will have a lot more time to perfect your application if you wait.
Applying under the Early Decision program is a huge commitment and is only recommended if you are one hundred percent sure you want to attend. The Early Decision program allows you to apply in November and hear back by mid-December. If you are accepted, you MUST attend the school. You may not attend anywhere else, and all other applications have to be withdrawn. The benefit is that you hear earlier and can show your commitment to the school. Most people do not choose this method because they like to compare financial aid packages from various schools before making a final decision. If you apply Early Decision, you may not apply early anywhere else.
There are two types of Early Action program that are offered by universities. The top three Ivy League schools offer Single Choice Early Action (SCEA), which allows applicants the benefits of Early Decision without having to commit to the school. Applicants can let the schools know if they are matriculating after they hear back from other schools they apply to. Regular Early Action allows you to apply to multiple schools early, but they all must be participants in the regular Early Action program. There are thought to be benefits to applying early (ED, EA, and EA2). The acceptance rate is higher for early programs than they are for regular admissions. However, some argue that this is the result of stronger applicants applying early. I would highly recommend applying under a SCEA program if you are confident that that school is one of your top choices. In 2013, Harvard University accepted more applicants from the early pool than from the regular pool.
Early Decision II
This is a program offered at a few universities, and is identical to the Early Decision program except you apply after you hear back from Early Action and Early Decision schools. You are committed to these schools similar to Early Decision schools.
Regular Admissions is the most common application timeline. The deadline is around early January, and gives you the most time to prepare your application. This is best for most applicants. If you think your first semester grades will help your chances, or are waiting for the results of a major competition you should wait for regular admissions. There is no point in applying early if you will be a better applicant in the winter.
Rolling admissions means you can apply at anytime and you will be notified as soon as they come up with a decision. This policy does not violate the restrictive early programs policies, so you can apply to a school with rolling admission and a different school with SCEA at the same time. (Applying to an international school will also not violate the early application agreements.) It is best to apply as early as possible with rolling admissions, as seats can fill up quickly in the freshmen class.