Candy

Candy is a consumable resource that can be used to power up and evolve your Pokémon. Candy is acquired by capturing Pokémon, or by hatching eggs. The type of Candy the player receives is specific to the Pokémon, i.e., capturing or hatching a Pikachu will give you Pikachu Candy. Transferring your duplicate Pokémon to Professor Willow will also give you Pokémon-specific candy, so if you're looking to evolve one, make sure to capture lots of duplicates.

Evolution
Candy is used to evolve your Pokémon that are capable of evolving. Evolving a Pokémon causes significant boost to CP. Evolving a Pokemon can cost 12 to 400 pieces of candy.