What is the NCAA?

The National Collegiate Athletic Association (NCAA, often said "NC-Double-A") is a voluntary association of about 1200 institutions, conferences, organizations and individuals that organizes the athletics programs of many colleges and universities in the United States. Its headquarters are currently located in Indianapolis, Indiana.

Links:

NCAA Eligibility
Excellent tool for understanding NCAA rules and regulations
NCAA Clearinghouse Online
The Online Resource for the National Collegiate Athletic Association