The Eye of Anubis Hologram is a foil symbol placed at the bottom right of all official Yu-Gi-Oh! cards, with the exception of some very early prints in the OCG such as those in Starter Box. Contrary to its appearance, it is not a sticker, but is printed directly onto the card.
The hologram serves as counterfeit protection as well as being used to identify the edition of a card. All Limited Edition and 1st Edition cards, in addition to being labeled as such on the card itself, will have a gold Eye of Anubis Hologram, while Unlimited Edition cards will have a silver one.
Official Proxy images of cards lack this hologram, either missing it entirely or replacing it with a solid square.