The GRE or Graduate Record Examination is the qualifying examination required to apply for colleges in the US for courses other than medicine (for which the USMLE is required ) and Management (for which the GMAT is required)
    (quick note : The SAT examinations that you could have heard about are qualifying examinations to be given after the 10 or 10+2 levels. Either way, they have nothing to do with you and I heartily suggest that you don't have anything to do with them. Living in India, after all, is not THAT bad  ;) )

Quite unlike all the major exams held in India, the GRE exam is held by a private company : ETS (Educational Testing Services ) . Consequently, Its much better managed than any of our exams are. The payment options are very flexible, there is no unnecessary documentation, the examinations are conducted on time, you get the results on time and in general, you can concentrate on the exam itself rather than on the attendant hassles.

The GRE exam concentrates mainly on the intellectual ability of the student (except of course for the verbal section). It is , in short , sort of an aptitude test.

After giving the GRE, a person is eligible to apply for a LARGE number of disciplines, just some of which we have tried to tell you more about in the "career" section.
However, it is to be noted that the GRE score is NOT the sole criterion deciding your eligibility to be admitted in a college of your choice in the US. Your earlier qualifications (which in your case would be a medical graduate with 5 1/2 years of graduate study) , your past research experience (if any ) in the field you are applying to , and finally the way you apply to the colleges also count. Probably more than our percentage obsessed minds will ever come to terms with...

What is the CAT? :

The CAT or computer adaptive test is the only way you can currently give the exam in India. Earlier there used to be a pencil and paper version of the test, which however has now been totally discontinued.

The CAT is an adaptive test. In other words as you start answering the easier questions, the computer presents you with more difficult questions. If you answer even these questions, you get even more difficult ones and so on..

Many people ( and most books) have listed techniques for getting good marks on the CAT by answering the earlier questions (which contribute significantly towards the score) more carefully and with a lot more time than the last questions, which they argue you should rush through without too much thought.
However I do not share this view. If you want to score in the 750 levels , I believe you should answer each question with the same amount of time, i.e.. you should start answering in at a proper speed right from the beginning.

For a contrasting ( and more widely accepted ) viewpoint, I have requested my friend Mukesh to write this article.