Extracurricular activities are immensely important when college admissions directors are evaluating applicants. Most people are aware of this but what many don't know is that more is not necessarily better.
Admissions directors want to see that you can do more than just test well. They want you to be more than a hermit who reads a lot and can recite the battles of the American Revolution in order. It is more or less taken for granted that if you are applying to college, you possess a basic knowledge of math, science, and history, and you can read and write. But can you balance all that studying and be true to your interests and drives?
Don't join every club your school offers, attend meetings, and think that's enough. Colleges want you to be deeply involved in your extracurriculars. Instead, join two clubs and hold offices in both. Compete with one sports team but work at it and excel. Most importantly, stick with your activities. It is far more impressive to see that you have been playing the trumpet for eight years and served as band captain as a senior, than to see that you ran track one year, then switched to baseball the next, then stopped playing sports all together.
Admissions directors are taking a chance when they accept you. They are risking the possibility that you lack the attention span to stick to a rigorous study schedule for four years. They are much more likely to let you attend their school if, in addition to a high grade point average and good test scores, you can show that you dedicated four years to your school swim team and were president of student government, for example.