The GMAT is a standardized test designed to test your ability to succeed in an MBA program. It is administered by the Graduate Management Admission Council.

The GMAT is broken down into three sections:
The GMAT can be an intimidating test. Hopefully, this website can help you prepare for the exam.
In addition to this site, there are a lot of other useful online resources, some of which are:
The official organization that offers the GMAT, The Graduate Management Admission Council is a leading advocate and resource for quality graduate schools of business worldwide. They provide information that helps business schools and students succeed.
Wikipedia, as usual, is a great source of free information about the GMAT, and provides a great overview for the Graduate Management Admission Test.
FREE GMAT Practice Test Questions provided by TestPrepReview are useful to get a feel for the types of questions you'll face on the GMAT.