GRE or Graduate Record Examination (GRE revised General Test) is a homogenized test taken by aspirants of graduate and business schools. It is a mandatory prerequisite for admission in most graduate schools and some business schools in USA and other English-speaking countries. It is a common yardstick to test the aptitude of prospective students of graduate and business schools who come from diverse educational and cultural backgrounds. GRE has a wide acceptance as it is accepted by huge number of business schools as well as graduate schools of various disciplines as well. The test offers a lot of facilities to applicants as the test can be taken multiple times, subject to a maximum number of five attempts, in a year. It can be taken in any one of more than 1,000 test centers across 160 countries. The best scores among the several attempts is accepted.

There are several subject GRE tests, notably in Mathematics, Physics, Psychology, Literature and more.  There was also a subject GRE in Computer Science, this had been revoked now. All subject GRE(s) test or measure test takers depth of understanding of particular subsets of that subject; the test contains a limited number of questions (in varying orders of difficulty) and should mostly be given if a test taker planning to pursue a PhD in the relevant field. The General GRE (or generalized) is mostly intended for MS applicants and it gauges the quantitative as well as the verbal aptitude of the applicant. Most PhD programs require/cite both the General as well as the Subject GRE as mandatory in their requirements for processing applications.

The GRE is held round the year at test centers around the world. The ETS website has a full list of testing centers. The test is computer-based, and paper-based where computer testing is unavailable. Paper-based testing is held three times per year.

The GRE is accepted by thousands of graduate schools and business schools. Test takers should research specific graduate programs to learn what tests they require.

A lot depends on a lot. Generally GRE scores are important but as someone has said more important if you score low than high. Some places automatically do not consider applications with low GREs, some don't. For physical sciences and math, GRE scores below 700 or even 750 would be considered low. Graduate admissions is still a very human enterprise. That means individuals will read the application and some care more about GRE than others. Some folks assign importance to GPA, others personal statements more than GRE, some not. And if you've had favorable contact with a particular professor or have a letter of recommendation from someone he/she respects that professor may well argue for admitting you even with less than ideal GRE scores. Keep in mind that none of this stuff predicts success in grad school very well (probably previous successful research experience works best), so arguments in admissions committee meetings over who to admit can have a passion which often accompanies decisions with working with flawed information and pet theories which have limited validity. So the only sensible answer is that at some places and in some fields it will matter more than in others.

There are some basic principles for people who are trying to prep on their own. Crafting studying technique is something more tactical, and personal, that you'll have to do on your own, based on what you know about how you best learn. Most of these, apart from distinctions as noted, apply to standardized tests in general:

• Start by creating a realistic schedule of daily work. Because the daily part is important, realistic probably translates to 15-20 minutes a day. "I'm going to spend two hours a night" is well-intentioned, but unlikely to happen.
• Use only the official materials. There is only 1 published book for the GRE.
• Materials include practice problems and practice tests. Do the practice problems first, then do regular practice testing as the actual test date approaches. There are two official GRE practice tests that are free to download. Because the tests are adaptive, they can be taken multiple times. You can get away with around three sittings per test without significant overlap, so that allows you six practice tests. These should be taken once a week for the six weeks leading up to the test, in an environment that simulates the actual testing environment as closely as possible (e.g., wake up and start the test at the times you would need to, isolate yourself in a distraction-free room, don't let yourself listen to music, put your phone somewhere else).
• Find someone with whom you can review the questions you get wrong. This doesn't have to be a professional if that's not feasible for you, but it should be someone who's good at the relevant content, or has scored well on the test before, or both.
• Do more practice problems for the content areas that seem to be surfacing the highest frequency of incorrect answers on your practice tests.
• Though there is no official vocabulary list, GRE prep requires studying vocabulary. Antonyms and analogies (and, to a lesser extent, sentence completions) lean much more heavily on sheer vocabulary than the passage or vocabulary in context questions that have largely replaced them on tests like the SAT for precisely that reason

Finally, just to underscore -- consistent, well-planned preparation is key. On tests like the GRE and the GMAT, cramming is basically a giant crap shoot.

The GRE is split into 6 sections: first an Analytical Writing section consisting of a 30-minute Argument Task and a 30-minute Issue task; then 2 sections each of Verbal Reasoning and Quantitative Reasoning, with a final "experimental" section that is either Verbal or Quantitative Reasoning. After the Analytical Writing section, the remaining sections occur in random order. Some tests also have an optional "research" section after the conclusion of the exam.

Two major changes to the GRE were affected in 2011: the scoring system was revised from a scale of 200-800 in 10-point increments to 130-170 in 1-point increments, and the computer testing algorithm was changed from a question-adaptive to section-adaptive. Before 2011, students who answered a series of questions correctly would be faced with progressively more difficult questions. After 2011, the first Verbal Reasoning and Quantitative Reasoning sections are of average difficulty, and the difficulty of the second sections is determined by performance on the first sections. Students who perform well will "level up" to a more difficult second section. The difficulty level of the test is taken into account when the score is tabulated, a process known as "equating".

Candidates are allowed to take the test up to five times within a rolling 12-month period with the restriction that one can take the test only once every 21 days.

The computer-based GRE has an adaptive algorithm that adjusts the difficulty level of the test based on the student's performance. The first Verbal Reasoning and Quantitative Reasoning sections are of average difficulty, with a mixture of easy and difficult questions. The difficulty of the second sections is determined by performance on the first sections. Students who perform well will "level up" to a more difficult second section. The difficulty level of the test is taken into account when the score is tabulated, a process known as "equating".