Most graduate schools require some sort of aptitude testing as part of the admissions process. Just as undergraduate schools require the SAT or ACT, graduate schools each have their own test.
DAT - Dental Admissions Test
Required for admission to dental school, the DAT measures a student's general academic ability, perception, and comprehension of scientific information.
The GMAT is used by business schools as part of the admissions criteria for MBA programs. The test is a standardized exam that measures a prospective student's verbal and math ability, as well as their analytical writing skills.
The GRE is a general test that measures verbal and quantitative skills, critical thinking, and analytical writing. It is used as part of the admissions process for most graduate school programs. While many programs require simply the general GRE, many programs require a subject exam. The GRE Subject Test is offered in biology; biochemistry, cell and molecular biology; computer science; chemistry; English literature; physics; math; and psychology.
LSAT - Law School Admissions Test
The LSAT measures a prospective law student's reading and verbal reasoning ability. It is an admissions requirement of all American Bar Association-approved law schools and most Canadian law schools. The test is a half-day standardized exam.
MAT - Millers Analogies Test
An hour-long exam, the MAT measures a student's analytical thinking ability. The MAT is often not required, but is an accepted alternative, by many graduate school programs.
The MCAT is an admissions requirement of nearly every medical school in the United States. The exam measures a student's ability to solve problems, think critically, and write, and it also measures one's knowledge of medical science concepts and principles. The MCAT is often used by veterinary schools, as well.
Required by all schools of optometry in the United States and several in Canada, the OAT is a general admissions exam that looks at a student's general knowledge as well as their scientific comprehension. A four part exam, the test includes questions regarding reading comprehension, quantitative reasoning, physics, and natural science.
The PCAT is a four-hour exam that measures the abilities of a prospective pharmacy student. The exam looks at one's general academic aptitude as well as knowledge of scientific principals used in the field of pharmacy.