The evolution system in Pokemon GO is a completely new concept. There are no PC boxes to be filled here, but candy is awarded for each capture! Evolving a Pokémon requires a certain number of candy, given when capturing or hatching Pokémon of the same family.
For example, evolving your Abra into Kadabra requires 25 Abra Candy. A standard catch nets you three candy, while using a Pinap Berry will give you six candy.
This means you need to capture between five and nine Abra in order to have enough Candy. The number of candy given when an Egg hatches is random.
Candy can also be used to "boost" your Pokémon, i.e. increase its CP. If you run out of candy, you can get some from Professor Willow by "transferring" any extra Pokémon you have from that family.
There are also Pokémon that can only evolve with the use of items. These were added with the arrival of Generation II.
Be aware that it is possible to come across already-evolved Pokémon in the wild. You may come across a Golduck, for example, but this one will be much weaker than if you evolve a Psyduck.
It is therefore important to capture as many Pokémon as possible, then evolve them!
Your Pokémon's CP will be modified as it evolves. The same goes for its moves, which will be randomly selected. You can find out all the information about battling gyms, Pokémon and attacks in the dedicated guides.
Written by Holomovement. Translated from the French by Millenium.