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I think you put those on the application yourself. It's like with a resume - you put additional classes you took, volunteering, some special skills (computer, extra languages), etc. to make yourself look better and more presentable. A college application i the same - you put some extra-curriculum activities to make yourself look like a good student as well as a person with many interests. You show you're active, so maybe the college will be able to use your skills in some teams, clubs, etc.

Depends on what the activities are and what colleges you are applying to. If you have some superb extracurriculars, of course those are going to help you and not harm you as far as college admissions is concerned. However, colleges have clearly learned of the dilution of quality extra curricular activities by the overly concerned high school students that do them only to help them get into college, and thus have reduce the impact of extra-curriculars on your admissions decision. Furthermore, if you are indeed way below the average accepted grades/test scores for a specifically college, what activities you do really will not help, when it comes down to it, the biggest factor is always grades, test scores, and to a certain extent the essay. Furthermore, obviously the more prestigious the school you apply to the high and denser the application will become with regard to grades and test scores, so for those schools extra activities become more important to set yourself apart from someone with identical grades to yours, but like I said, you must first attain the grades before your extra activities can begin to have an effect. Overall, these extra activities will never hurt you, but make sure you do them because you really care, not because you need them to get into college, because if you are only doing them for college you can never truly say you've put your best effort in and tried to make any more difference than what is required to get credit for doing such an activity.

It'll be one of the questions on your application. Some schools also will have them on your transcript. And yes, you can put almost all of your activities on your application (sometimes just in list form).

you put them on the application.