The ASVAB also known as the Armed Services Vocational Aptitude Battery is a test that measures your aptitude which is how well you do something.  A common mistake by most people is thinking that it is a measure of you IQ. Many people either forget this or don’t know this and they get worked up about the exam.  Saying that, it is also important to take a ASVAB sample test to insure you know what type of questions will be asked.    Another thing to keep in mind is every person in the armed forces has to take the exam and you are not being called out to take it. 
Note: While the U.S. Navy is older than the Marine Corps,[50] the Marine Corps takes precedence due to previous inconsistencies in the Navy's birth date. The Marine Corps has recognized its observed birth date on a more consistent basis. The Second Continental Congress is considered to have established the Navy on 13 October 1775 by authorizing the purchase of ships, but did not actually pass the "Rules for the Regulation of the Navy of the United Colonies" until 27 November 1775.[51] The Marine Corps was established by act of said Congress on 10 November 1775. The Navy did not officially recognize 13 October 1775 as its birth date until 1972, when then–Chief of Naval Operations Admiral Elmo Zumwalt authorized it to be observed as such.[50]
What is the military? In simple terms, the U.S. Armed Forces are made up of the five armed service branches: Air Force, Army, Coast Guard, Marine Corps, and Navy. There are three general categories of military people: active duty (full-time soldiers and sailors), reserve & guard forces (usually work a civilian job, but can be called to full-time military duty), and veterans and retirees (past members of the military). And of course there are the millions of family members and friends of military members, past and present. But you're here to learn more about the military. There is much to learn! So first the basics. Exactly who is in charge?
Examinees also receive a score on what is called the Armed Forces Qualification Test (AFQT). AFQT scores are computed using the Standard Scores from four ASVAB subtests: Arithmetic Reasoning (AR), Mathematics Knowledge (MK), Paragraph Comprehension (PC), and Word Knowledge (WK). AFQT scores are reported as percentiles between 1-99. An AFQT percentile score indicates the percentage of examinees in a reference group that scored at or below that particular score. For current AFQT scores, the reference group is a sample of 18 to 23 year old youth who took the ASVAB as part of a national norming study conducted in 1997. Thus, an AFQT score of 90 indicates that the examinee scored as well as or better than 90% of the nationally-representative sample of 18 to 23 year old youth. An AFQT score of 50 indicates that the examinee scored as well as or better than 50% of the nationally-representative sample.