Pal Park is a new feature introduced in Pokemon Diamond and Pearl. It allows players to trade Pokemon from 3rd Generation Pokemon games to Pokemon Diamond and Pearl when the 3rd Generation game is inserted in the Nintendo DS's GBA slot. It is currently the only way to get the starters and legendaries of Kanto, Johto and Hoenn into the 4th Generation. Located on Route 221, Pal Park is only accessible after a player has first obtained the National Dex.
Pal Park is similar to the Time Capsule in Pokemon Gold, Silver and Crystal since it allows you to transfer Pokemon from previous generations to the current generation, however, unlike the Time Capsule, Pokemon transferred through Pal Park are removed from 3rd Generation Pokemon games and are permanently in Pokemon Diamond and Pearl. Similar to the Time Capsule, Pokemon can't know any HM moves from the 3rd generation if you want to transfer them through Pal Park.
Unlike previous games, Pokemon you trade will not just appear in your party, you have to actually catch them. Once you've selected Pal Park from the main menu of Pokemon Diamond or Pearl, you will now be able to catch them in Pal Park through an event known as a Catching Show. Once you capture a Pokemon you received from a trade, it should have exactly the same stats and trainer information as it had before, including OT, ID number, held item, and PokeBall. It will also display the region that it was originally captured in and the level it was transferred over at. For example if your friend traded you a LV50 Pikachu with 100 speed points, those will be its stats with no difference.
How do you catch Pokemon in Pal Park? It's simple. Firstly, this place is kind of like the Safari Zone. Unlike the Safari Zone though, you're given special Pokeballs that have a 100% catch rate. The downside is that you only get six and you must catch any Pokemon you run into. You are also limited to only transferring six Pokemon within 24 hours per GBA cartridge, and no further Pokemon can be transferred until the previous six Pokemon have been caught. At the start before you enter Pal Park to catch Pokemon, you choose the 6-transferred Pokemon to appear in the park for you to catch.
In Pal Park, there are many areas like water, and grass. Say for example I was trying to catch that Pikachu from before; you have to think, where would it most likely be living in? It won't be living in the water, but maybe in the grass. If you think you've found the area, don't start celebrating just yet because you might be in the wrong place. Some Pokemon are also very rare.
If you manage to catch all the Pokemon you selected, you'll receive a special Berry the Park Ranger. What type of Berry depends on how quickly the Pokemon are caught and the rarity of the transferred Pokemon.