Deck modification cards... modify the deck where cards are drawn from, or a/the discard pile. The name gives the meaning of the card away, but there is more to them than just that, there are two types of deck modification cards
there are deck modification cards that mess with the system of drawing cards at the beginning of each person's turn, this isn't direct deck modification but Hey! these cards rock. They are usually made to obtain a card that the maker wants by allowing them to draw out of the discad pile or draw from anywhere in the deck. They may also allow a person to chose their card from the deck. These cards should have a limit on how many turns that the effect lasts. so that one player isn't dominating because they can play any card that is in the game.
another type of deck modification cards are ones that add/ take away cards from the deck, (e.g. discard all cards in the deck that have pictures of animals) There are even some that swap the draw and discard piles. These cards can force other players to put valuable cards that they have into they draw pile. Allowing them to be played by another person. Or a deck modification card may just make one player shuffle the deck.
you get the idea.