Almost all exams test the same general areas:

Verbal skills
Quantitative skills
Writing skills

Some tests require field-specific questions. The GRE, for example, offers “Subject Areas” such as Biochemistry, Biology, Chemistry, Computer Science, English Literature, Mathematics, Physics, and Psychology.

Think twice before taking any optional exams or optional portions of exams. If it is optional, not taking it cannot be held against you, but if you do take it, a poor score can weaken your application. In general, take whatever exams you need, but only what you need. The exceptions to that guideline are if you are an exceptional test-taker or if you think the score might strengthen your application.