Just as with the algebra and arithmetic sections of the GRE, the geometry questions in the math section of the exam are meant to measure your knowledge of basic, simple geometry that you probably learned in middle school or earlier. Sounds simple, right?
Again, it's simple, but deceptively so -- the GRE is infamous for making easy questions look hard.
In order to answer correctly the geometry questions on the GRE, you'll need to know the basics about the following concepts, which are explained in the Geometry tutorials section:
You've probably heard of these concepts already, and probably know a trick or two about some of them. If that's the case, then great; but that doesn't mean that you should skimp on this section.