The Pokémon Peninsula > Pokémon Fire Red/Leaf Green
As suggested by the titles, Fire Red and Leaf Green are remakes of the original Pokémon games, Red and Green. They feature better graphics and more characters.
For the English-language release, Leaf Green remains Leaf Green (it wasn't renamed to Water Blue, which is definitely a good thing). These games are for the GameBoy Advance system, meaning that they will work on GameBoy Advance and above only. Most Pokémon can be found in both games, but there are a number that can only be found in one or the other. To catch all possible Pokémon and complete the Pokédex, Pokémon must be traded using a Game Link Cable between two players, or using the GBA Wireless Adapter that comes bundled with the game.
FR/LG will be pretty familiar to you if you played the original Red or Blue/Green. The story and events are mostly the same. There are, however, a few new areas to access (the Sevii Islands), and a new feature is wireless play. Using the bundled Wireless Adapter, you can battle or trade wirelessly, berry powder with friends and hang around in the Union Room. You can't catch Pokémon that weren't in the original 150, but after you beat the Elite Four you can get the NationalDex which will let you trade between FR/LG and Ruby/Sapphire.
Pokémon in Each Version