The Armed Forces Vocational Aptitude Battery, more commonly referred to as the ASVAB is used by the Army primarily for two purposes: (1) to determine if you have the mental capability to be sucessful through basic training and other Army training programs, and (2) to determine your aptitute for learning various Army jobs.
The ASVAB consists of nine subtests: General Science, Arithmetic Reasoning, Word Knowledge, Paragraph Comprehension, Mathematics Knowledge, Electronic Information, Auto & Shop, Mechanical Comprehension, and Assembling Objects.
The ASVAB comes in two flavors: The pencil and paper version, and the computerized version. If you're taking the test as part of your enlistment process into the Army, you'll most likely take the computerized version during your trip to MEPS.
The Armed Forces Qualification Test (AFQT), often mistakenly called the "overall score," is actually comprised from only four of the subtests (Arithmetic Reasoning, Word Knowledge, Paragraph Comprehension, and Math Knowledge). The other subtests are used to determine job qualifications.
If the MOS (job) you reserved requires learning a foreign language, you'll also take the Defense Language Aptitude Battery (DLAB).