Pokémon Stadium is a game for Nintendo 64, released by Nintendo.
The basic purpose of the game is simple: It allows all fans to take those simple but exciting Pokémon fights from the GameBoy game, and present them in Glorious 3D.
The game offers a lot of different match choices. First, it comes with a Transfer Pak, a thing that's connected to the N64 controller allowing you to use any Pokémon from your Pokemon Red & Blue game. Pokémon Stadium will also emulate the Super Gameboy (but only for Pokemon Red & Blue!) and let you play the game on a TV screen, also optionally at higher speed.
Of course, it allows you to also choose "rental" Pokémon so the GameBoy game isn't required at all - but it's much more fun in GB way.
The game has different "cups" for Pokémon of different levels, and also has a 2-player mode and a "free battle" mode where everything goes, and up to 2 players can fight against the computer player or each other.
It also includes an improved Pokédex and item/Pokémon storage management. (This is really great for those who play the GB game.)
It also has series of minigames for up to 4 players (One of my favorites is Sushi-Go-Round).
All in all, a really interesting Pokémon fight game.
A side note: Porygon's appearance in this game is probably the most gross waste of N64's rendering capability =)