It's the Graduate Record Examination, and used after obtaining an undergraduate degree in order to "prove" oneself capable of meeting the academic rigor of various graduate level programs.
The GRE is used for graduate studies in sciences and mathematics heavy programs. The LSATs are required for entry into law school (stands for something like the law school admissions test), and the MCATS are required for entry into medical school.
Basically, it's a racket that gouges your wallet and stresses ya out! 
Can ya tell I'm just about done with law school and totally burned out on the whole process?