The Graduate Record Examination is your ticket into graduate school. The test measures verbal reasoning, quantitative reasoning, critical thinking and analytical writing. The test lasts 3 hours and it can take 2-4 weeks to receive full results. I strongly suggest taking practice exams before paying 160 dollars to take this exam. Some gradate programs do not require a minimum GRE score for acceptance, but the better the score the greater your chances of getting accepted to the program.
Here are some websites that you may find useful:
Master Your GRE
Free Practice Test