The Visa and MasterCard associations do not issue credit or debit cards directly. They are each collectively comprised of thousands of banks worldwide. These banks pay membership fees to the associations, and are
thus permitted to issue cards.
American Express, Diners Club, and Discover are not associations;
each of these companies issues its card directly, and maintains
its own clearance network.
MasterCard is accepted at over 18 million locations worldwide.
Visa Web Site