Starter Pokémon, or Starters, are the Pokémon a trainer chooses at the beginning of their Pokémon journey.
In each generation of Pokémon games, a set of Starter Pokémon are available for new trainers to choose from in sets of three; the types that make up these sets are Fire, Grass, and Water. Each starter Pokémon has three evolutionary forms. Pokémon Yellow is currently the sole exception to the above, with Pikachu as the only starter available to the player.
Bulbasaur and Rowlet are currently the only Starters to be dual-types in their basic forms, as aside from both of them being Grass-types, Bulbasaur's also a Poison-type while Rowlet's also a Flying-type.
All the second and third evolutions of the fire starter Pokémon from generation III to V have Fighting as a secondary type. They are also the only Fire/Fighting type Pokémon.
Typhlosion's currently the only final evolution of a Fire-type starter to not be a dual-type.
In Unova, the Starters also gain signature attacks unique to them: Snivy gained Leaf Tornado, Oshawott gained Razor Shell and Tepig gained Heat Crash. Leaf Tornado and Razor Shell were given to other Pokémon in later generations.
All Starter Pokemon have Hidden Abilities, such as Greninja with Protean.
Except for Pikachu and Eevee, all the Starters' final evolutions can learn the ultimate attacks by move tutor upon reaching maximum happiness. Grass-type starters can learn the ultimate Grass attack Frenzy Plant; Fire-type starters can learn the ultimate Fire attack Blast Burn; Water-type starters can learn the ultimate Water attack Hydro Cannon.
Both Eevee and Pikachu can learn what can be called the ultimate attacks in their respective type in any form. Pikachu can learn Volt Tackle, and Eevee, in any of its evolved forms, can learn Hyper Beam.
The rival trainer the player character comes across will always pick the player's starter's weakness, with a few exceptions.
Sceptile gains Dragon-type as a secondary type on Mega Evolving, making it the only Mega Evolved starter to gain a new type (Charizard was already dual when the Mega X version replaces Flying with Dragon).
In Omega Ruby/Alpha Sapphire, Professor Birch will give Torchic/Treecko/Mudkip to save him from the wild Pokémon. After beating the Elite Four and the Delta Episode, he will give the player the choice of Chikorita/Totodile/Cyndaquil to rescue him again. After beating the Elite four a second time, he will give the player the choice Turtwig/Piplup/Chimchar. After beating the Elite Four for the third time, he will give the player the choice of Snivy/Oshawott/Tepig. Thus, the player can receive one of the three regional Pokémon for Hoenn, Johto, Sinnoh and Unova before beating the Elite Four four times.
In Pokémon X and Y, two starters can be obtained. At the beginning of the game, the player can choose among Chespin, Fennekin, and Froakie, then, a little while later, among the Kanto starters Bulbasaur, Charmander, and Squirtle.
The player can later catch the first evolved forms of both the Kanto and Kalos starters in some Grass, Fire, and Water Friend Safaris. This marks the first time any Pokemon related to starters have been available to catch in the wild.