Generation refers to the Pokémon game series. It is a group of games that were released at or around the same time. It also means that games in the same generation are compatible with the others, containing the same Pokémon and the number of moves there are to be learned. For example "Pokémon FireRed and LeafGreen", though set in the same time period and region as "Pokémon Red and Blue", are not considered to be Generation I games. This is because they were released years after the original and have many new features and differences, which makes them incompatible with the original. Since they have the same features as "Pokémon Ruby and Sapphire" and were released around the same time, they are considered Generation III games.
Along with this, games in different generations have different features and/or game mechanics.
Throughout each generation, there is usually a theme to the region introduced in a specific generation. As a result, each region and generation have several key features, such as important Pokémon and places, that place emphasis on those themes. In particular, the very last Pokémon introduced in that region and in that generation embody that generation's theme in some way. Here is the list of themes that are associated with the generations and regions:
Generation I (Kanto): Science: The starters of the Kanto region, Bulbasaur, Charmander, and Squirtle, are all based on animals that existed on Earth millions of years ago (mammalian reptiles, salamanders, and turtles respectively). Eevee (the Evolution Pokémon) and its first three eeveelutions were introduced here. There is a museum in Pewter City where fossil Pokémon may be revived. Porygon, the Virtual Pokémon, is introduced in this generation. Mewtwo is a genetics experiment gone wrong in a similar vein as Frankenstein. The generation's final two Pokémon, Mewtwo and Mew, embody the two main aspects of science that are covered in that region: technology and genetics respectively.
Generation II (Johto): Cultural Traditions:
Generation III (Hoenn): The Environment:
Generation IV (Sinnoh): Mythology:
Generation V (Unova): Duality:
Generation VI (Kalos): Life, Death and the Ecosystem