Add-ons or expansions (also called expansion packs) are content created for games that is released separately from the main game release. Downloadable content (DLC) is a specific form of add-on distributed through the internet.
The first game in the Fallout series to feature downloadable content was Fallout 3, with a total of 5 add-on packs released:
• Broken Steel
• The Pitt (add-on)
• Operation: Anchorage (add-on)
• Point Lookout (add-on)
• Mothership Zeta (add-on)
Fallout: New Vegas also features add-ons, with a current total of 4 pre-order bonuses and a single add-on pack as of yet:
• Dead Money.