An airline program for frequent fliers from Delta Airlines.
It also has attached to it branded Credit Cards and other perks. (see: http://www.americanexpress.com/apply/cgi-bin/page.cgi?24/2115/b/71)
You can also get more Sky miles from affiliated non-airline partners such as car rentals and hotels.
The airline(s) use it to drive more traffic to themselves and their partners.