There is yet another incarnation of the GRE that most people do not have to take, but is still important. That is the GRE Writing assessment. If a departmental requirement, a prospective graduate student will take this test to see if they can: propose arguements, write a coherent discussion, and be in command of the English language. This test is not a requirement of all grad schools, (I never took it) but its use is becoming more widespread.
A special note concerning the Analytical portion of the GRE general test. Even though someone may thing they are analytical in nature and do not need to prepare for that portion of the test, THEY DO! The type of questions in that section of the test are never seen in a "normal" education. They involve cruise ships leaving on different days for different length trips on Mondays and Fridays only and other problems that require extensive diagrams to complete. If I had not purchased a study guide for that section of the test, I surely would have got a poor score, but by being prepared for the type of questions to come, I ended up with a 790 Analytical and a 98+ percentile. Highly recommended.