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.
Fallout 3
The first game in the Fallout series to feature downloadable content was Fallout 3. With a total of five add-on packs released:
Fallout: New Vegas
Fallout: New Vegas also features add-ons, with a current total of two pre-order bonuses and six add-on packs.