What is GRE test ? and what are requirments for this exam ?

The Graduate Record Examinations is a standardized test that is an admissions requirement for many graduate schools in the United States and Canada. The GRE is owned and administered by the Educational Testing Service. The test was established in 1936 by the Carnegie Foundation for the Advancement of Teaching.
GRE Exam Eligibility
The only eligibility requirement for GRE is the document required to prove your identity. In India, you’ll need a valid passport. You’ll have to get your original passport (not photocopies) which clearly shows your full name, photograph and signature.
No other documentation (like birth certificate, international driving license etc) can be used as an alternate identity proof. They are very strict about this.
Apart from this eligibility requirement, ETS does not set any age, qualification, timing related pre-requisites. However, there will be a range of eligibility criteria set by universities that accept GRE scores. For each of their programs, they may have minimum age, experience and qualification related filters.