The Grass type, sometimes called the "Plant type", is one of the 18 canon types in the Pokémon games. One of the "original 15", it's the embodiment of plants and nature, and all Pokémon and moves of this type will involve plants in some way. It also appears to be the embodiment of fungi, as mushroom based-Pokémon tend to get thrust into this type; this may reference older scientific literature that classified fungi into the Plantae kingdom before breaking them off into their own.
It has a mostly-exclusive egg group, which was also known as "Plant" in older literature.
It also appears in the TCG, with Bug and Poison Pokémon often being lumped in with actual Grass types. (Newer expansions have put Poison Pokémon into the Psychic type instead.)
This table has been correct since Generation 1, with the only change being to add the Steel type.
Grass types are naturally immune to Leech Seed. Starting in Generation 6, they're also immune to any powder-based moves, such as Stun Spore, Sleep Powder, and Poison Powder, moves that coincidentally have wide distribution among them.
Some commonly-given fan justifications for this setup:
- Grass's advantages: Plants need water to grow, and that water typically comes from the ground or rocks.
- Bug/Flying on Grass: Most bugs eat plants. Similarly, one of the Flying type's embodiments is of birds, and many birds eat plants.
- Flying's other embodiment is wind, and when the National Weather Service (or your country's equivalent) issues a wind advisory or high wind warning, it's generally on the grounds that the wind is strong enough to blow down trees, resulting in blocked roads, power outages, and damage to your house.
- Fire on Grass: Forest fires.
- Ice on Grass: Many plant species can't stand cold.
- Poison on Grass: An environmentalist message? Many plants can't stand poison.
- Electric on Grass: Ground is immune to Electric, and many real-life plants are rooted in the ground.