Normal - Basic pokémon that don't necessarily have an affinity with any other type as their primary characteristic, only secondary. Mostly based on real life animals. For instance, a giraffe who happens to have some psychic powers is the Normal/Psychic Girafarig.

Grass - Flora (flowers/wood/grass/mushrooms etc.)

Fire - Burning substances (fire/lava/magma etc.)

Water - Either controls water or lives in a watery environment and has some sort of affinity to it (living in water or coming from water alone doesn't necessarily make you a water type).

Electric - Affinity with electricity and general energy (such as plasma).

Ground - Either controls one or more of the many types of natural soil or has ease in traveling through it (such as through digging).

Rock - Minerals and gems (rocks/stones/crystals etc.) note: Diamond is hard enough that it is usally ascribed to the Steel type for its better defensive properties.

Flying - Flies or floats as one of its main means of locomotion or has an affinity with the wind.

Ice - Has an affinity with ice/is capable of using moves that greatly lower the opponent's temperature, even if the pokémon itself isn't made of ice or lives around it.

Psychic - Pokémon with higher mental prowess that possess abilities related to their mind (hypnosis/telekinesis etc.)

Dark - Mostly brutish, mean and territorial pokémon that mistreat others or aren't above cheating to win. Alternatively, pokémon connected to some sort of bad energy such as bad luck.

Fighting - Pretty much the normal type but with an interest and inclination towards combat. These are pokémon who don't like standing still and enjoy fighting, but unlike the Dark type, aren't inclined to using dirty tactics to win. The good version of the Dark Type, you could say.