A YDK file contains a Yu-Gi-Oh! deck used by various Yu-Gi-Oh! card dueling programs, such as YGOPRO, YGOPRODeck, Duelingbook, and Yu-Gi-Oh! Dueling Nexus. It stores multiple passcodes that correspond to a deck of virtual playing cards in the Yu-Gi-Oh! game. The card dueling game then displays the appropriate life points, counters, and effects for the cards.

The YDK deck file may contain passcodes for Monster, Spell, and Trap cards that can be loaded into the Yu-Gi-Oh! game to duel other players. Once the YDK file is loaded in the program, the application matches the passcodes to the card information in its database, then displays it to the user. Some examples of card information include the level, attack, and defense.