There are two nation-wide tests available to be administered to most highschool students, the SAT (Scholastic Aptitude Test) and ACT (American College Testing). As I understand it, the SAT is preferred by the more elite universities and on the coasts, while the ACT is sufficient for the middle of the country and state universities (I only ever took the ACT). Either is required by most universities.

I've never worked in univeristy admissions so I can't say how much the tests count for, but I'm pretty sure it's a significant determining factor for acceptance of an application (along with highschool grades and entrance letters, letters of recommendation, etc).