Stickers is a way that Deck Heroes lets you quickly sort through Creatures you have encountered in your battles in the Explorer's Gate, Trials, Gauntlet and more. This function can be accessed from the Top-Right Drop Down Menu, under the Menu section by clicking the Stickers Icon. This is a great tool within the game to help you identify and track down cards that you'd like to incorporate into your deck.
However, remember that the stickers only show creatures and heroes which you have or have encountered in a battle. In order to view every creature (from within the game), go to a runed creature, press either it's vitality, protean, or violence runes (if it has one of those equipped), then press "Creatures" at the bottom of the screen. You will then be taken to an interface which will show you every creature in the game. The same can be done for heroes (any hero rune can be used).
The Deck Heroes Wiki uses a similar setup to allow you to quickly browse Heroes and Creatures, appreciate the Artwork, and learn more about these cards so you can craft the perfect decks.