The ASVAB is a series of timed aptitude tests that are used to classify selected candidates into appropriate job roles as well as ultimately decide the eligibility of candidates for US military service.  In other words, perform poorly on this test and you could severely limit your opportunities in the military or even prevent yourself from serving.

As you go along, keep in mind that the ASVAB practice test is just that: practice. Memorizing these questions and answers will not be very helpful on the actual test because it is unlikely to have any of the same exact questions. If you only know the right answers to the sample questions, you won’t be prepared for the real thing. Study the concepts until you understand them fully, and then you’ll be able to answer any question that shows up on the test.
The ASVAB currently contains 9 sections (except the written test, which contains 8 sections). The duration of each test varies from as low as ten minutes up to 36 minutes for Arithmetic Reasoning; the entire ASVAB is three hours long. The test is typically administered in a computerized format at Military Entrance Processing Stations, known as MEPS, or at a satellite location called a Military Entrance Test (MET) site. The ASVAB is administered by computer at the MEPS, while a written version is given at most MET sites. Testing procedures vary depending on the mode of administration.[3]
The Kaplan guide has everything I needed to prepare myself for the exam. The book was a great help for refreshing my memory in math and English for the test. Timing yourself on the practice tests is also a great way to make sure you are ahead of the game and never worry about time in the actual test. This is one of the best study guides I have ever seen or used but be prepared to spend time doing the work. As I said it will take hours to go through the material and learn what you need. I believed this book had a direct result in scoring an AFQT of 99. Excellent Book!
The ASVAB was created in 1968. By 1976, all branches of the military began using this test. In 2002, the test underwent many revisions, but its main goal of gauging a person’s basic skills remained the same. Today, there is a computerized version of the test as well as a written version. The Department of Defense developed this test and it’s taken by students in thousands of schools across the country. It is also given at Military Entrance Processing Stations (MEPS).
The Electronics Information section of the practice test gauges your knowledge of electrical equipment and parts, including circuits, currents, batteries, and resistors. An example may be, “Because solid state diodes have no filament, they: don’t work, are less efficient than tubes, require less operating power, or require more operating power?” The CAT-ASVAB has 16 questions in 8 minutes; the paper-and-pencil version has 20 questions in 9 minutes.
The ASVAB is a timed test that measures your skills in a number of different areas. You complete questions that reveal your skills in paragraph comprehension, word knowledge, arithmetic reasoning and mathematics knowledge. These are basic skills that you will need as a member of the U.S. military. The score you receive on the ASVAB is factored into your Armed Forces Qualifying Test (AFQT) score. This score is used to figure out whether you qualify to enlist in the armed services.
The AFQT score is the most important ASVAB score, because it determines if you can enlist in the U.S. Army. However, the U.S. Army also converts the ASVAB test scores into 10 other composite score areas known as "line scores" that determine what MOS an individual may qualify for. Listed below are the parts of the ASVAB that affect your AFQT test scores and each of the ten line scores.
Jump up ^ The United States Coast Guard has both military and law enforcement functions. Title 14 of the United States Code provides that "The Coast Guard as established 28 January 1915, shall be a military service and a branch of the armed forces of the United States at all times." Coast Guard units, or ships of its predecessor service, the Revenue Cutter Service, have seen combat in every war and armed conflict of the United States since 1790, including the Iraq War.
Word Knowledge (WK), Paragraph Comprehension (PC), Arithmetic Reasoning (AR), and Mathematics Knowledge (MK). Scores on the AFQT are used to determine your eligibility for enlistment in the Army, Navy, Air Force, or Marine Corps. The other six test scores on the ASVAB tests are used to determine the best job for you in the military as your grades will demonstrate knowledge, skill, and interest in certain subjects and tasks. 

General Science tests the ability to answer questions on a variety of science topics drawn from courses taught in most high schools. The life science items cover botany, zoology, anatomy and physiology, and ecology. The earth and space science items are based on astronomy, geology, meteorology and oceanography. The physical science items measure force and motion mechanics, energy, fluids, atomic structure and chemistry.

Varsity Tutors’ Learning Tools free ASVAB study material is a great opportunity for you to tune up your skills before you take the test. You might discover that you simply have to brush up on some science, math, or definitions to get ready to take the test. The simple ASVAB review tests enable you become more confident as well. There’s no better way to succeed in reaching your goals than by being prepared ahead of time.
Battlefield commission: under certain conditions, enlisted personnel who have skills that separate them from their peers can become officers by direct commissioning of a commander so authorized to grant them. This type of commission is rarely granted and is reserved only for the most exceptional enlisted personnel; it is done on an ad hoc basis, typically only in wartime. No direct battlefield commissions have been awarded since the Vietnam War. The Navy and Air Force do not employ this commissioning path.
After adding so much information to the aptitude test, there was a bit of difficulty interpreting the test results. In addition to that, a vast majority of test takers were deemed as being under qualified based on their test results. This is why the percentile change was made. It ensured that a 50% actually correlated with a person doing better than 50% of the test takers. Those revisions have worked hand-in-hand with the preparation of the armed forces.