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. However, Pokémon Yellow is currently the sole exception to all the above, with Pikachu as the only starter available to the player.
In Unova, the Starters, with the exception of Tepig, Pignite and Emboar, originally had signature attacks, but were also given to certain other Pokémon: Snivy possesses Leaf Tornado, Oshawott possesses Razor Shell and Tepig alone possesses Heat Crash.
This also not always the case. For example, one may breed a starter Pokémon and get one with a special typing (For example: Greninja can have Torrent (normal) or Protean (special) however this is a rare occasion. Same with Delphox it can have Blaze (normal) or Magician (special) besides it being the ability of Hoopa)
Except for Pikachu and Eevee, all the Starters in any form can learn the ultimate attacks. Bulbasaur, Chikorita, Treecko, Turtwig, Snivy, Chespin and Rowlet can learn the ultimate Grass attack Frenzy Plant. Charmander, Cyndaquil, Torchic, Chimchar, Tepig, Fennekin and Litten can learn the ultimate Fire attack Blast Burn. And Squirtle, Totodile, Mudkip, Piplup, Oshawott, Froakie and Popplio can learn the ultimate Water attack Hydro Cannon.
However, both Eevee and Pikachu can learn what can be called the ultimate attacks in there respective type in any form. Pikachu can learn Volt Tackle, and Eevee in one of its evolved forms, can learn Hyper Beam.
The starters also have the Pledge moves that are exculsive to them: Bulbasaur, Chikorita, Treecko, Turtwig, Snivy, Chespin and Rowlet have Grass Pledge, Charmander, Cyndaquil, Torchic, Chimchar, Tepig, Fennekin and Litten have Fire Pledge and Squirtle, Totodile, Mudkip, Piplup, Oshawott, Froakie and Popplio have Water Pledge.
The rival trainer the player character comes across will always pick your Pokémon's weakness, with a few exceptions.
Thus far, all Fire-type starters have a flame somewhere on their bodies by their final evolution. Three of them, Charmander, Cyndaquil, and Chimchar, have such a flame from the beginning.
In the first six Generations the vast majority of fully evolved Fire type starters end up dual-type with Typhlosion being the sole exception, however three out of six Grass type starters and three out of six Water type starters never end up dual-typed.
Starting in Generation III with Ruby/Sapphire/Emerald the starters began to be obtained outside the resident professor's lab with Hoenn having the player character save Professor Birch from a wild Pokémon by picking from Treecko/Torchic/Mudkip in his briefcase, the wild Pokémon was a Poochyena in Ruby/Sapphire and a Zigzagoon in Emerald. For Generation IV in Diamond/Pearl the Player and their rival find Sinnoh's starters Turtwig/Chimchar/Piplup inside a briefcase left behind at Lake Verity's shore by Professor Rowan, though in Platinum Dawn/Lucas returns it to Rowan shortly after he stops them from walking into the tall grass just outside Twinleaf Town giving them out on the spot. In Black/White Professor Juniper delivers a present to the player character's house containing the starters Snivy/Tepig/Oshawott. Black 2/White 2 had Bianca giving them to the player character in Aspertia City, their home town, from inside a container. In Kalos the player character receives one of the regional starters, Chespin/Fennekin/Froakie, from a kid named Tierno in Aquacorde Town at an outdoor table along with several other kids, and they also get a Kanto starter from Professor Sycamore.
In Omega Ruby/Alpha Sapphire, Professor Birch will give Torchic/Treecko/Mudkip to save him from the wild Pokémon. After you beat the Elite Four and the Delta Episode, he will give you the choice of Chikorita/Totodile/Cyndaquil to rescue him again. After beating the Elite four a second time he will give you the choice Turtwig/Piplup/Chimchar. Now beating the Elite Four for the third time, he will give the choice of Snivy/Oshawott/Tepig. That means you get 1 of the 3 regional Pokémon for Hoenn, Johto, Sinnoh and Unova before you beat the Elite Four 4 times.
In Pokemon XY, 2 starters can be obtained. You pick between Chespin, Fennekin, and Froakie, then later you can choose all of the Kanto Starters except Pikachu and Eevee.