(the following info is from www.nintendo.com)

The American version of Pokémon Stadium is actually based on the second Japanese edition of Pokémon Stadium, which is currently in the development stages in Japan. Pokémon Trainers will be able to conduct turn-based battles similar to the Game Boy versions of Pokémon, but outrageous animation sequences and flashy special attack effects will make your Pokémon come to life like never before.

One of the most exciting aspects of Pokémon Stadium is that you'll be able to battle using your most loyal Pokémon from the Red and Blue versions of Pokémon. Thanks to a special adaptor that plugs into the bottom of your controller, data can be transferred straight from a Game Boy cartridge into your N64. The TMs, attacks and levels of your trained Pokémon will be available on Pokémon Stadium, so you won't have to leave any of your favorites behind.

On top of the visual treat of watching your Pokémon duke it out on the big screen, Pokémon Stadium will also contain a variety of mini-games starring all types of Pokémon. It's currently unclear exactly how these miniature challenges will relate to the organized Pokémon battles, but any time you get four Clefairies together you're bound to have fun.

Even though we have trouble reading the Japanese, these early screen shots demonstrate the game's unique fighting stadiums Each of the 151 Pokémon can be found in Pokémon Stadium, even if you decide not to transfer data from your Game Boy Color Pokémon games. Pokémon Stadium will allow up to four players to compete in a grueling Pokémon tournament, designed to reveal which of your friends is the best Pokémon Trainer.