The Graduate Record Examination (GRE) is a 3.9 hour, multiple-choice, multi-stage test required by most graduate schools. It's run by the Educational Testing Service, the same people who run the SAT.
Schools differ in how they use your GRE score. Some consider it very important, while others view it as a formality. We recommend asking your prospective programs — most will be quite willing to tell you what part the test plays in their admissions decisions.
The GRE consists of an Analytical Writing section that contains two essay questions, two Verbal Ability (vocabulary and reading) sections that contain 20 multiple-choice questions, and two Quantitative Ability (math) sections that contain 20 multiple-choice questions.
There is also a sixth, experimental section that will be either Verbal or Quantitative. This portion will not count towards your final score — ETS uses it to test questions for use on future exams. Unfortunately, you'll have no way of knowing which part is experimental (it will look identical to the real Verbal or Quantitative section), so you'll need to do your best on the entire test.
The GRE is a multi-stage test. This means that the computer will use your performance on one section to determine the difficulty level of the next section. Within a section, however, the question selection is static and you can skip around. If do very well on your first verbal section, for example, the second verbal section you will see will be much more difficult. This is a good thing, however, because you must get to and successfully tackle the hardest questions to get to the highest possible scores.
You will receive separate Verbal and Quantitative Scores; these are reported on a scale from 130 to 170 in one point increments. The Analytical Writing section is listed separately, and is scored on a scale of 0 to 6, in half–point increments.
The GRE Subject Tests are similar to SAT IIs in that they test your knowledge of a particular subject like chemistry or literature. Not every school requires a GRE subject test, but many of the most competitive programs do. ETS offers the tests three times a year — they are not part of the standard GRE.