The newest type — and the one that finally broke Dragon.
Fairy was added in Gen 6 (X & Y, 2013) specifically to counter the oppressive Dragon-heavy metagames of Gens 4 and 5. It is immune to Dragon, super-effective against Dragon, and resists Fighting, Bug and Dark. Pokémon that had existed for years were retroactively retyped — Clefairy, Jigglypuff, Mr. Mime, Azumarill and others all gained the Fairy typing in the Gen 6 patch, and overnight became competitively relevant.
Fairy is a defensive pivot and Dragon counter. Clefable's Magic Guard + Unaware sets hard-wall most offensive Dragons. Tapu Koko, Tapu Lele, Tapu Bulu and Tapu Fini (the Alolan Tapus) are all Fairy-types and dominated Gen 7 VGC. Mimikyu's Disguise ability guarantees one free turn of setup.
Moonblast (95 BP, 30% Sp. Atk drop) is the iconic Fairy special move. Play Rough is the physical equivalent (90 BP, 10% Atk drop). Draining Kiss (50 BP + 75% HP recovery) is the recovery option.
Sylveon was the first Eeveelution added in six years — between Gen 4 (Leafeon, Glaceon) and Gen 6 — and Game Freak specifically designed the Fairy type around Sylveon's reveal. The type announcement and Sylveon's announcement happened in the same Nintendo Direct.