Stickers were a new feature introduced in Pokémon GO Version 0.177.0. These allowed for you to attach stickers to the Gifts that you send to other people. These stickers are all aesthetic and don't provide any benefits.

You can get stickers by opening gifts and through purchasing them in the Pokémon GO Shop.

Pokémon GO Stickerss

List of Stickers

Pikachu Pikachu Gift
Pikachu Pikachu Gift
Pikachu Pikachu Gift
Pikachu & Raichu Pikachu & Raichu Gift
Ditto Ditto Gift
Squirtle Squirtle Gift
Charmander Charmander Gift
Bulbasaur Bulbasaur Gift