Pokemon Yellow is basically the same as Pokemon Red / Blue except for a few changes. They are:
-In the beginning, Prof. Oak fights/captures a wild Pikachu, and gives it to you.
-Gary starts off with an Eevee
-Team Rocket is in!!! They have Meowth, Ekans, and Koffing (all of which evolve)
-It's possible to catch wild Charmanders, Squirtles, and Bulbasaurs (like the show)
-Cheats don't work in this version
-Pictures of some Pokémon and Gym Leaders have been improved
-You can print your Pokedex and Pokémon stickers via the GameBoy Printer
-Pikachu follows you around (you can see him on the screen)
-Follows the show more closely than Blue/Red
-Pokémon evolve at different levels in the Yellow version
-Some TMs won't work on the Pokémon that they used to work on in the Red/Blue version