Go to
http://www.ncaa.org and click on "About the NCAA".
From the Overview section:
'
The "membership" or "members" -- The colleges, universities and conferences that make up the NCAA. The members appoint volunteer representatives that serve on committees which introduce and vote on rules called bylaws. The members also establish programs to govern, promote and further the purposes and goals of intercollegiate athletics.
The "national office" -- Approximately 350 paid professionals that implement the rules and programs established by the membership. The national office staff is located primarily at the headquarters office in Indianapolis, Indiana.
The "Association" - The entire organization comprised of members and staff. Many believe the Association rules college athletics; however, it is actually a bottom-up organization in which the members rule the Association.'
I also find the section titled "History of the NCAA" to be useful when considering why the NCAA exists.