Assassin's Creed: Anthology is a collector's edition consisting of the main games in the Assassin's Creed series prior to Assassin's Creed IV: Black Flag.
Packaged in a special presentation box, the anthology includes Assassin's Creed, Assassin's Creed II, Assassin's Creed: Brotherhood, Assassin's Creed: Revelations, and Assassin's Creed III. The games are contained in a steelbook, with three discs; Assassin's Creed and Assassin's Creed II, Brotherhood and Revelations, and Assassin's Creed III.
The anthology contains all downloadable content for each respective game. Assassin's Creed II contains the DLC on-disc, as it is the "Game of the Year" edition. The other games contain redemption codes that access the DLC from the internet.
In addition to the games, the Anthology also contains five A4 art prints.
Source: Assassins Creed Wikia, "Assassin's Creed: Anthology", available under the CC-BY-SA License.