According to the ETS website, "The Graduate Record Examinations® (GRE®) General Test measures verbal reasoning, quantitative reasoning, and critical thinking and analytical writing skills. The GRE® Subject Tests gauge undergraduate achievement in 8 specific fields of study." (See Reference 1 below.)
You can take the computer-based or paper-based GRE General Test. The exam is approximately three to three and three-fourths hours with four scored sections and one to two unscored research sections. (See Reference 2 below.)
Subject Tests give graduate programs a sense of your subject expertise. As of 2009, Subject Tests are only paper-based, and they are offered in eight subjects, including biochemistry, computer science, literature and psychology. (See Reference 3 below.)
GRE General Tests are offered worldwide year-round, and Subject Tests are offered in April, October and November. You can register for the GRE on the ETS website, by phone or by mail. (See References 4 below.)
You can study for the GRE with practice questions online or in test preparation books. Test preparation centers and private tutors provide courses and personalized GRE tutoring.