What Is A Pokédex?
A Pokédex is a digital encyclopedia that categorizes Pokémon you catch. It puts them into a virtual catalog which you can easily access to learn more about the Pokémon you have. You can see what species your Pokémon are and if it has the opportunity to evolve.
A nice feature of the Pokédex in most Pokémon games is to sort the list by Alphabetical order, number, regions, etc.
This image is for Pokémon Go, this shows all of the possible Pokémon (upon it’s current release state as of August 2016). Only Pokémon you’ve caught will show, the rest will be simple shadows.