SAT
One of the many tests students commonly take on their road to college is the SAT. It is administered by the College Board and scored by the Educational Testing Service. Each college may use SAT scores in very different ways - with some colleges preferring the ACT to the SAT. So, think about where you want to go to college and do some research as to what test they use and how they use it.
The College Board has a website that is loaded with information about the SAT. They have specific portions of their site dedicated to students, parents, and teachers.
Also, there is a large amount of concise information related to the SAT on Wikipedia's site:
SAT History
SAT Structure
SAT scoring