Pokémon Leaf Green is a remake of the first Pokémon game the Blue Version.
Story:
The story is the same as in the Blue Version. You're a Pokémon trainer who goes on  his adventure through the world of the Pokémon and wants to become a member of the Hall of Fame.
Gameplay:
Pokémon is like an RPG. You choose one of 3 Pokémons at the beginning and later you catch other Pokémons with Poké Balls. You fight with these Pokémons against other trainers and wild Pokémons. For each Pokémon you beat your own one gets experience, gains levels, learns new attacks and evolves. The fights are round-based: you choose an attack (one of the many different ones) then you attack and then your opponent attacks. In the fights you can use items to heal and cure your Pokémons.
Next to the fights you can talk to a lot of people who give you more information about the world and you can collect many many items and Pokémons. There are 150 Pokémons (in this game) you can collect and fight with.
Graphic:
The villages and the world are the same as in the Blue Version but on the graphical level of the Ruby and Sapphire Versions. As well as the graphic there are things like the map which are adopted from the Ruby and Sapphire Version. The graphic itself is fantastic. It's probably the best graphic on GBA. The villages look great and the Pokémons on the Pokédex and in the fights are great animated.
Sound:
The sound is also the same as in the first game. There are some memorable tunes which will not leave you for a long time.
Conclusion:
Pokémon Leaf Green is a great made remake of the Blue Version with the same story and Pokémons from the original but with the graphic and details from the Ruby and Sapphire Version. At the end it's still a great game with cool fights a big world and a paradise for people who like to collect as much as possible. One of the best GBA games!

It looks like Pokemon prevails once again. Games like these give very good reason why Pokemon is as successful as it is today. The remakes are especially entertaining.