Pal Park is a location at the east of Route 221 where you can migrate your Pokémon from the previous versions of the GBA games in Generation III. It can only be accessed after you have defeated the Elite Four. When you first visit Pal Park, you will meet Prof. Oak, and he will give you a National Pokédex.
In order to migrate your Pokémon, go to the title screen (where there are the Continue and New Game button) and select "Migrate Pokémon" with a Pokémon cartridge of the earlier versions (Ruby/Sapphire/Emerald/FireRed/LeafGreen) inserted in your Nintendo DS. The Pokémon shown in there are the Pokémon you store in the PCs of the GBA catridges. When migrating, the Pokémon from the GBA cartridge will be gone. Only 6 Pokémon at a time can be migrated. After migrating the 6th Pokémon, you need to wait another 24 hours if you want to migrate again with the same game park. Changing the time on your DS will delay another 24 hours of migrating.
Next, the migrated Pokémon will be found in the Pal Park. Head to the Pal Park and the person will give you some special Poké Balls equivalent in number to the Pokémon you migrated. Then head to the grassy areas or the water areas to find them. The special Poké Balls have a 100% catch rate. The time taken for you to catch those Pokémon is also recorded. Pal Park is also found in Pokémon SoulSilver and HeartGold. It is found in Fuchsia City in Kanto, where Safari Zone used to be. The Pal Park in those versions looks and works the same way as in Diamond and Pearl, but the 24 hours migrating limit has been removed.
Pal Park may also be used to get all the starter Pokémon from the GBA games, since you don't have to beat Elite Four in those games to migrate Pokémon to newer games. You just have to capture 6 other Pokémon, making you have 7 Pokémons. You then just put 6 of them into your PC, and you can now migrate to newer games. Then you would just start a new save and do it again with another starter Pokémon. Also, in Black and White, you can migrate Pokémon from Diamond, Pearl, Platinum, HeartGold, and SoulSilver. The player will have to have two DSes, one with a Generation IV game, and one with a Generation V game in it.