Oracle

In the ancient world, oracles were temples where people would go to get their fortune told. Unlike modern fortune-telling, everyone involved took that shit seriously, and people would pay large sums of money to listen to a woman, probably high on various poisonous fumes, ramble cryptically at them.

Pathfinder

Sometimes, in Golarion, a god will attempt to divinely inspire a prophet to go forth and do their will among the people. This divine power gives them many powerful abilities, but the message is usually a little garbled, and every oracle is a little off in the head. It also warps them a little: some go lame, others go blind, a few go mad, and all sorts of other side effects. It's a bit of a mixed bag for the god in question.

The sorcerer to the cleric's wizard, oracles are divine spellcasters that cast from a pool of select spells spontaneously, rather than preparing them in advance. Like sorcerers, they lose out to clerics on pure versatility, since they can't turn undead and have access to fewer spells, but, also like sorcerers, they get more spells and access to a mystery, which is like a cleric's choice of god but much more badass. It's their equivalent to the sorcerer's bloodline powers, and gives them a ton of new class abilities: the mystery of metal, for instance, grants the ability to wear heavy armor and wield martial weapons on top of metal-related orisons. (That's the pretentious name for oracle spells, incidentally.)

Pathfinder made the spontaneous casters viable choices, and oracles are powerful and a lot of fun to play, especially because you get to roleplay what essentially amounts to a crazy homeless person with magical powers, and maybe a heart of gold if you like. They're also excellent for dual-classing with the Sorcerer if you want to go for a Mystic Theurge, because then you can just max the fuck out of your Charisma score instead of juggling two abilities.

The Classes of Pathfinder 1st Edition
Core Classes:	Barbarian - Bard - Cleric - Druid - Fighter - Monk Paladin - Ranger - Rogue - Sorcerer - Wizard
Advanced Player's Guide:	Alchemist - Antipaladin - Cavalier Inquisitor - Oracle - Summoner - Witch
Advanced Class Guide:	Arcanist - Bloodrager - Brawler - Hunter - Investigator Shaman - Skald - Slayer - Swashbuckler - Warpriest
Occult Adventures:	Kineticist - Medium - Mesmerist Occultist - Psychic - Spiritualist
Ultimate X:	Gunslinger - Magus - Ninja - Samurai - Shifter - Vigilante