1. There are 5 DLC packs, Operation Anchorage, The Pitt, Broken Steel, Point Lookout, and Mothership Zeta (which is being developed at the moment).
2. They are only available through CRAP-TACULAR Windows Live. However, Op Ancorage and the Pitt, and Broken Steel and Point Lookout are being released on discs, and a GotY edition is planned with all five DLC packs. The DLC will be coming to the PS3 and 360 too, though only after Mothership Zeta is finished. The only way to get Zeta on disc is to get the GotY edition.