There are two types of GRE tests, general and subject. The general test is taken by every student in any major who plans to go to graduate school. The subject test is specific to your major or particular subject. For example, there is a subject test in mathematics. Most universities will require your general GRE scores for admission to their graduate program.
Many schools also require a subject test.
It is suggested that this test be taken during the fall of your senior year while the material is still fresh in your mind. The GRE tests are offered four times a year, usually in October, December, April and June. Deadlines for registration are usually a month or two prior to the exam, so be sure to check deadlines. Note that on the final date in June, only the general test is offered and no subject tests. Since graduate programs often have deadlines as early as January or February, the GRE exams should be taken in October or December of your senior year.