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.
Bulbasaur's the only Starter to be dual-type in basic form, being both a Grass- and a poison-type.
All known Grass-type starters are based on pre-historic creatures and evolution. Venusaur has legs that lift it off the ground (a characteristic of dinosaurs), Meganium is based off an Apatosaurus, Sceptile is based on a Dilophosaurus, the Snivy-Serperior evolution represents the loss of hind legs (the transition from dinosaurs to snakes) It could also be based off the Gigantophis, a dinosaur that was also serpetine, and Chespin is based off the first mammal, the Megazostrodon. Torterra is based off a Tortoise. Also, each have a type of plant on their bodies which grows bigger through evolution.
With the exception of Typhlosion, most Fire-type Starters' final evolutions consist of two types.
Except for Pikachu and Eevee, all the Starters in any form can learn the ultimate attacks. Bulbasaur, Chikorita, Treecko, Turtwig, Snivy and Chespin can learn the ultimate Grass attack Frenzy Plant. Charmander, Cyndaquil, Torchic, Chimchar, Tepig and Fennekin can learn the ultimate Fire attack Blast Burn. And Squirtle, Totodile, Mudkip, Piplup, Oshawott and Froakie 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 and Chespin have Grass Pledge, Charmander, Cyndaquil, Torchic, Chimchar, Tepig and Fennekin have Fire Pledge and Squirtle, Totodile, Mudkip, Piplup, Oshawott and Froakie have Water Pledge.
The rival trainer you come 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 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 & a Zigzagoon in Emerald. For Generation IV in Diamond/Pearl the Player & 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 players house containing the starters Snivy/Tepig/Oshawott. Black/White 2 had Bianca giving them to the player in Aspertia City, their home town, from inside a container. In Kalos the player 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, & they also get a Kanto starter from Professor Sycamore.