A type is what makes Pokemon unique in their own way. For example, Normal is weak against Fighting. And Dragon is weak against Ice, Fairy, and itself, Dragon. Here is a list of all the Pokemon types. The ??? type was only used for one move, Curse, but was later changed to a Ghost-type move. The only ???-type Pokemon is Arceus.
Normal
Psychic
Fire
Electric
Water
999 (Glitch type)
Bird (Glitch type)
Steel
Grass
Fighting
Dragon
Dark
Poison
Ice
Rock
Flying
Ghost
Fairy
Bug
Ground
???