From Bulbapedia, the community-driven Pokémon encyclopedia.
Pokémon Land (Japanese: ポケモンランド Pokémon Land) is an attraction located on the Island of the Giant Pokémon. It appeared in Island of the Giant Pokémon.
In the anime
The park's biggest attraction is its many different robotic Pokémon. When Ash, his friends and Team Rocket ended up in Pokémon Land, they didn't realize that the Pokémon were robots, and believed them to be giants. They learned they were robots when they were trying to escape them. A roller coaster car they were riding in crashed through a robotic Zapdos, sending metal flying everywhere. Sadly, the amusement park was destroyed.
Giovanni spent billions units of currency on building Pokémon Land, and wasn't happy to learn that everything had been destroyed.
For unexplained reasons, the robotic Pokémon seemed to try to kill the Trainers and the lost Pokémon, as evidenced by Zapdos and Moltres discharging electricity and flames at a fleeing Ash, Misty and Brock and Jessie and James, respectively, Charizard trying to vaporize Ash and friends with a Flamethrower, Blastoise spraying the group's Pokémon with a water blast that would be a lethal dose, Kabutops chasing Jessie and James, Charizard, Blastoise, and Venusaur chasing down the lost Pokémon, and Rhydon apparently trying to attack the lost Pokémon.
In the dub for Attack of the Prehistoric Pokémon, James indicated that the payoff they'll get from selling Fossils will be such that the Dinosaur theme park they'll open will rival that of the theme park (called "PokéWorld" here).
Pokémon seen at Pokémon Land
In the manga
In the Pokémon Pocket Monsters manga
In the Pokémon Pocket Monsters manga, a similar attraction to Pokémon Land known as the Amusement Park was built by Giovanni. Red, Clefairy and Pikachu visited the Amusement Park. Giovanni was not happy to see them in the place he built. He sent out Machamp and Poliwhirl, which evolved into Poliwrath, to take care of them. Like most places that Clefairy visits, this one ends up being destroyed in the end by Clefairy himself.