Let me answer this as briefly as possible:
the what's:
As you no doubt know, the GRE stands for Graduate Record Examination.
Any fundamental queries regarding the exam would be answered from their home site www.gre.org. I recommend that you take a through tour of the site.
the who's:
As the name suggests, people who take the exam are usually, though not limited to the Undergraduate section of the student community. Anyone who decides to pursue a Master's or Doctoral course in the United States of America would have to take it up at the earliest.
the why's:
The exam is meant to measure the abstract reasoning skills of the students who have completed or are pursuing their undergraduate degrees to a reasonable extent. It is also supposedly a good indicator of the performance of the student in her first year of graduate studies.
the how's:
This will require a comprehensive answer which I will be posting later.
