In pokemon yellow ash gets pikachu as his starter pokemon. Besides that ash even gets the three standard starters of kanto on his journey as does red in yellow. Another important things are that pikachu dislikes pokeballs and does not evolve.
Pokemon Yellow was the third game released in the USA, following on from the revolutionary Pokemon Red and Pokemon Blue. Nicknamed "the Pokemon Anime game", Pokemon Yellow bought to the Gameboy the fun of a Pokemon RPG with a storyline that resembled the popular cartoon show.
As said before, the game copies many of the cartoon's elements. Gym Leaders will use Pokemon that they used in the Anime, and you might spot a few familiar trainers waiting for battle in the vast plains of Kanto - look out for AJ from "The Path to the Pokemon League", Joe and Giselle from "The School of Hardknocks" and even the infamous Team Rocket - Jesse and James, armed with their very own Meowth, Ekans and Koffing!
Another striking difference is that Yellow has Nurse Joy in the pokemon centres and Officer Jenny in many cities. Because in the anime The Team Rocket trio was given the leadership of Viridian city as the gym leader was away, the player gets to battle with the pokemon that the trio were given.