It is a test that is required to be taken to enlist in the United States Armed Forces. Your score decided what branch, and then MOS, that you are qualified for.
But it can serve a purpose beyond that. It is an aptitude test as well. So instead of paying a ton of money to take an aptitude test to see where your interests are, you can take the ASVAB. Its a useful test to take no matter where your life may lead you.
