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.
Paizo's attempt to port over, buff up, and grimdark over the Favored Soul, in a nutshell. Like most full spontaneous casters, they are Tier 2, capable of breaking the game with the right (or wrong) spells, but can't trivialize everything thanks to their low number of spells.
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 their spell-preparing counterparts on pure versatility, since they can't channel energy 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 spells. They also suffer from an oracle's curse, like going blind or having lots of grumpy ghosts constantly knocking shit out of your hands. While it usually comes with significant mechanical drawbacks (even something like Tongues, which restricts the languages you can speak in combat, cuts off [language-dependent] spells and summoning), the curse also unlocks new spells and powers for the oracle in question (though some of the less painful ones also don't get much in return), and as they rise in level and learn to live with it it becomes less and less of a liability.
One little utilized difference from Cleric is that, unlike Clerics, Oracles aren't prohibited from casting spells opposite of their deity's alignment. Most of these spells are useless if you aren't the matching alignment, as they only work against opposite alignment or do strictly evil things like dooming a soul to Hell regardless of alignment. This means the primary use for this is playing good aligned Oracles cursed by evil deities without losing access to [Good] spells. There are a few gems you can pick up though like Infernal Healing, Screaming Flames, Hellfire Ray (if you take merciful spell), Death Clutch and undead creation stuff for good Oracles or Angelic Aspect and Burst of Radiance for evil Oracles.
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|
|Core Classes:|| Barbarian - Bard - Cleric - Druid - Fighter - Monk |
Paladin - Ranger - Rogue - Sorcerer - Wizard
| Arcanist - Bloodrager - Brawler - Hunter - Investigator |
Shaman - Skald - Slayer - Swashbuckler - Warpriest
| Alchemist - Antipaladin - Cavalier |
Inquisitor - Oracle - Summoner - Witch
| Kineticist - Medium - Mesmerist |
Occultist - Psychic - Spiritualist
|Ultimate X:||Gunslinger - Magus - Ninja - Samurai - Shifter - Vigilante|
| Aegis - Cryptic - Dread - Marksman - Psion |
Psychic Warrior - Soulknife - Tactician - Vitalist - Wilder
|Path of War:||Harbinger - Mystic - Stalker - Warder - Warlord - Zealot|