Candy

Candy is a consumable resource that can be used to power up and evolve your Pokémon. Candy is acquired by capturing Pokémon. The type of Candy the player receives is specific to the Pokémon that was captured, i.e., capturing a Pikachu will give you Pikachu Candy. Transferring your duplicate Pokemon to Professor Willow will also give you Pokemon 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 from 25 to 50 pieces of candy.