A Pokédex is a high-tech encyclopedia given to each trainer, when they become an official, licensed, Pokémon trainer. It is given out by different people in the different generations of Pokémon games.
Who Gives It To Me?
In Generation IV, Professor Rowan is the one who gives you your Pokédex along with your first Pokémon. This Pokédex has 151 Pokémon in Diamond and Pearl, however in Pokémon Platinum there are 210 Pokémon, following the addition of such Pokémon as Giratina to the regular Pokédex. However the National Dex has 493.
This Pokédex was a very simple one. It had three Options: Data, Cry, and Area. The functions will be explained better at the end.
This Pokédex had a couple more additions to it. The functions available were Data, Area, Cry, Print, Sort, and Search. The functions will be explained at the bottom.
There were even more functions available in this version. They are, Menu, Search, Data, Area, Cry, and Size (1).
This Pokédex is, so far, the most advanced Pokédex known to the Pokémon World. It has the most functions, which are, Search, Switch Pokédex (only available when you receive the National Dex), Data, Area, Cry, Size (2), and Forms.
Data: Data is the information that the Pokédex offers on the selected Pokémon.
Area: This option shows you the map of the current region, along with markers where you can find the selected Pokémon.
Cry: This option lets you hear the cry of the selected Pokémon.
Print: This option was only available in the second generation of Pokémon, and it let you print the selected Pokémon via a connection with a GameBoy Printer.
Sort: This option allowed you to sort the Pokémon in your Pokédex. In G/S/C there was; New Pokédex which is all the G/S/C Pokémon. Old Pokémon which is all the R/B/Y Pokémon. And A to Z Mode which simply sorted Pokémon according to their name. And in R/S/E and in D/P/P it allows you to order your Pokédex by Numerical Mode which lists them by number, A to Z Mode, Heaviest Mode, which lists the Pokémon in order of their weight, Lightest Mode, which works the same as Heaviest Mode, except reversed, Tallest Mode, which shows the Pokémon in order of their height, and Smallest Mode which works the same as the Tallest Mode, but reversed.
Search: This option allows you to search the Pokédex by Name, Color, and Type.
Menu: This option gives you four choices, Back To List which closes the menu, List Top, which takes you to the first Pokémon, List Bottom, which takes you to the last Pokémon, and Close Pokédex, which backs out of the Pokédex completely.
Size (1): This is the first version of the Size function, it was available in R/S/E, it shows the height of your Pokémon in comparison to you (the trainer).
Size (2): This option is available in D/P/P and it allows you to see the height of the selected Pokémon in comparison to your trainer, as well as the weight of the selected Pokémon in comparison to your trainer.
Switch Pokédex: This option is available in D/P/P. It allows you to switch between the Sinnoh Dex and the National Dex.
Forms: This option is available in D/P/P. It allows you to see the different forms of the selected Pokémon. Some Pokémon have more forms than others, though the common ones are simply, Male and Female forms. Some other forms are Wes Sea and East Sea, which are available with Shellos and Gastrodon. There are other forms available as well.