Pokemon Red and Green, are the first Pokemon games, of the Pokemon series.
When you start a new game, you'll be greeted by professor Oak. Who'll tell you about the World of Pokemon. After he tells you, you can name your character (who is known as "Red" in the canon series), after you name your character, Oak tells you about your rival, who is his grandson. After he tells you, you can name your rival (who is known as "Blue" in the Canon, and "Gary" in the Anime). After that, you start the game. Pokemon are a big part of the gameplay, as you can't leave pallet town without one (without cheats/gltiches). The main thing in the gameplay is to collect pokemon, and defeating gym leaders to earn badges (and later beat the elite 4 and your rival).
Pokemon Blue Edit
Pokemon Blue is a lot like Pokemon Red and Green, but with updated graphics and bug fixes.
Pokemon Yellow Edit
A lot like Red and Green, except the story is more based on the Anime.
Firered and Leafgreen Edit
Remakes of Red and Green on the Game Boy Advance, with a lot more added.