Tribute cards pay homage to the best baseball card sets of all time. It's a way to celebrate old designs with new players. As well as new designs with old players. And vice versa.
How does it work? Well, I start by finding a set that I like. From there, I get a high quality scan of an existing card. Then, I take that scan and break the card apart into Photoshop/Pixlr/Vectr/whatever layers. At that point, I create a new baseball card that has never existed! For instance, here you can see the 1952 Topps design with Brad Hawpe.