A recipe card is made by combining three other cards already in your Pack. These three cards can all be the same or different, but they will all be part of the same collection (with a few exceptions), and they will generate a new card that is also part of that same collection. Sometimes, a recipe will require a card that can only be made through another recipe, and that recipe might require another card that can only be made through a recipe. Recipes are made by clicking on the red pot icon from your Pack.
Please note that recipe cards are never released to any regular buy/sell markets for purchase! However, you can buy them already made from the player-run Fort Worth or Moscow auction market, steal recipe cards from other players and Rats, trade other cards for them, or get lucky and find one by drawing a card or playing FlipRat. Recipe cards can also be bought and sold in the Fort Worth auction market, but this is entirely dependent on what players happen to be auctioning at the time.
For more information on Recipe cards, please read the following article.