The Tomb Keepers are a group of Egyptians from ancient times, a clan with a single purpose. Their duty, throughout all of history, was to protect the tomb of the nameless pharaoh and its legacy. This legacy is carved into the back of each Tomb Keeper (tattooed in the English dub) - a carving of the Tablet of Lost Memories, in preparation for the day that the pharaoh would be reincarnated and be shown his memories once more. It was said that once the pharaoh returns, the Tomb Keepers have no more necessary purpose to exist.
In the manga, the clan broke into different factions over the rivalry of the Millennium Items. One faction was Shadi's faction, who watched over the Eye, Ring, Scale, and Key. The Eye and Ring were given to Pegasus and Bakura, respectively, leaving the Scale and Key.
The other faction of the Tomb Keepers (who appeared in both the anime and manga) were Marik Ishtar's mother and father. When they are introduced to the series, the mother appears to be barren, which could result in the eventual death of the Tomb Keeper line and the end of the care-taking of the tomb. As a result, Odion was adopted into the family, and began undergoing the education of the life of a Tomb Keeper from infancy.
However, several years later, the mother gave birth to Ishizu and Marik Ishtar, and, as the oldest son, he took Odion's place as the next Tomb Keeper, guarding the legacy of the Nameless Pharaoh in his place. However, it was never a role he desired, resulting in his eventual murder of his father (sent to the Shadow Realm in the English dub) and the development of Yami Marik as a dissociative personality.
Later, in the final arc, Ishizu and Marik take Yami Yugi and Yugi Muto to the tomb for the Ceremonial Duel, where the Pharaoh is released, ending the duty of the Tomb Keepers for all time (this did not mean the members were killed, as Marik originally believed).