Rise of the Tomb Raider: 20 Year Celebration is a re-release Rise of the Tomb Raider, made available almost a year after the game's original release. It contains:
Rise of the Tomb Raider (base game)
Rise of the Tomb Raider: 20 Year Celebration Pack (DLC)
Artbook (retail preorder version only)
It also contains all of the previously released DLC, including:
Rise of the Tomb Raider: Baba Yaga - The Temple of the Witch
Rise of the Tomb Raider: Cold Darkness Awakened
Rise of the Tomb Raider: Endurance Mode
35 expedition cards
12 outfits
7 weapons