Similar to a PDA in our reality, this device was invented by in game character Professor Samuel Oak a respected Professor in the science of pokémon.
The device was designed to catalogue and provide information regarding originally 150 species of pokémon.
As with the further development of the series starting with Pokémon Green, Pokémon Red and Pokémon Blue, the device itself was updated and changed in the game to appear more high tech and to gather more information than ever including the new total of 386 unique species of pokémon.
Each Trainer in the game is issued a blank device and must collect information about each species that the player encounters in the game which is easilly done simply by finding such a species, or capturing one by either traditional methods of a battle or by trade.
Each region in the Pokémon world favours its own Regional Pokédex as a default setting but honours the National Pokédex as the official standard.