This article is from the Digital Broadcast Satellites FAQ, by Brian Trosko firstname.lastname@example.org with numerous contributions by others.
DBS services broadcast across the entire nation. Obviously, not
everyone subscribes to the same programming. The access card is
what authorizes your decoder to decode the channels you are
subscribed to. This access card is remotely programmable by your
program provider, and it is this which allows you to change channel
subscriptions without too much of a hassle. In short, the
access card is what makes the system work.