Think a Divine Magic Sorcerer, only with some "Totally not Jesus as a Class" features, including holy wings (or unholy ones because favored souls can just as easily be evil). Its spontaneous casting would probably be a much bigger deal if Clerics didn't already have the ability to spontaneously cast cure/inflict spells; however, it can still cast more spells per day than a cleric, which is a huge deal when your party is full of chaotic stupids who can't resist the urge to get themselves seriously maimed. Despite the fact that heavy armor doesn't interfere with divine spellcasting, the Favored Soul also sacrifices heavy armor proficiency for some bullshit damage reduction and elemental resistances.
It's similar to a War Domain cleric, but without the ability to turn/rebuke undead. Since turning undead is what fuels Divine Metamagic and the rest of the cleric's obscene brokenness, Favored Souls have been described as "nerfed clerics".
The Favored Soul's primary weakness is that, if played as a gish class as intended, it is slightly MAD. Being of a more martial bent than non-war-domain clerics, it relies more heavily on strength; lacking a cleric's heavy armor proficiency, it must make up for that with greater dexterity; its spells known and spells per day are determined by charisma, but the difficulty class of saving throws against its spells is determined by wisdom. Of course, a lot of this can be avoided by playing them as straight casters rather than a gish, and/or by sticking to spells that don't require saves (such as buffs).
One of the Favored Soul's more unusual characteristics is that it has all good saving throws, enabling a Tiefling or Aasimar to get Fiendish or Celestial Bloodline at ECL 2 instead of ECL 4, and Outsider Wings at ECL 4 instead of ECL 7. The only other class with this property is the Monk, which sucks huge amounts of ass. Although getting Outsider Wings would be a waste of the extremely similar wings that Favored Souls get at level 17, nobody should be dumping 17 levels into Favored Soul in the first place. Any Aasimar or Tiefling players who scoop up Favored Soul at level 1 just for fast access to Outsider Wings should consider jumping to a prestige class that continues the Favored Soul's spellcasting progression, like a Church inquisitor, Contemplative, Sacred Exorcist, or Void Disciple.
5th edition decided to stick its oar in by trying to bring back the Favored Soul as a new Sorcerous Origin, a subclass for Sorcerer. In fact, this Mystic Theurge-theme underwent the most revisions of any single subclass in the game so far!
The first version of the 5e Favored Soul appeared in an Unearthed Arcana article on class designing. Gets one Cleric Domain's special spells as bonus spells to their list, free proficiency with basic arms & armor, attacking twice per Attack Action, flight at-will by sprouting and dismissing wings, and the ability to heal themselves by casting any Domain spell. Do remember that you only get access to your Domain's bonus spells, though, not to the Cleric spells as a whole, so only Favored Souls of Life can fill the healbot role. It's not a bad idea, but it's often held up as problematic because so many of these traits are supposed to make it more like the cleric (aka, a healer gish) without addressing the critical error in that - namely, with a mere d6 hitdie, sorcerers are not the kinds of casters you really want to get into melee.
An alternate take on the Sorcerer-as-Favored-Soul appeared in the February 7 Unearthed Arcana article on Sorcerers. This one is a lot less "spontaneous caster cleric" in nature, possibly due to the Theurge tradition for Wizards. Instead, it's perhaps closer in nature to the 4e Invoker. Its Divine Magic class trait lets it learn Cleric spells as well as Sorcerer spells, although still kind of hindered by the low number of spells that Sorcerers get. At 1st level, it gets Supernatural Resilience (+1 to max HP per level) and Favored By The Gods (1/encounter, add +2d4 to the result of a failed saving throw or missed attack roll), which makes it beefier than the typical Sorcerer, but without expecting them to actually get into the fray, which they aren't really suited for. At 6th level, it picks up the ambiguously as fuck worded feature Blessed Countenance, which basically doubles Proficiency bonus on Charisma checks. The 14th level Divine Purity feature grants immunity to poison damage, the poisoned condition and disease, and at level 18, they finish up with Unearthly Recovery where they can heal half of their missing max hit points when below 50% health 1/day.
And a third Favored Soul Sorcerer appeared on May 1st, 2017; this one goes for more of a heal-bot approach. Divine Magic, acquired at 1st level, means it can learn Cleric spells and Sorcerer spells at will, and gets a "free" Cure Wounds spell - an automatically learned spell that doesn't use up its limited supply of spells known. It also retains Favored By The Gods, as per the last version. At level 6, it gains Empowered Healing, allowing it to spend sorcery dice to reroll the dice results for healing spells it casts. Level 14 gives it access to Angelic Form, where it can summon or dismiss ethereal wings that grant it a Fly speed of 30 feet. Finally, at level 18, it gains Unearthly Recovery, which lets it regain 50% of its hit points after dropping to half or less of its HP once per long rest.
It was this third version that WoTC decided was the best option. With a few tweaks - specifically, that "free Cure Wounds spell" was changed into the player's choice of Cure Wounds, Inflict Wounds, Bless, Bane or Protection from Evil & Good, "Angelic Form" was renamed "Otherworldly Wings", and "Empowered Healing" can be used to boost the healing spells of allies within 5 feet as well as the sorcerer's own - the angel-blooded sorcerer became an official option in "Xanathar's Guide to Everything", where it was given the official name of Divine Soul.
If the Sorcerer path doesn't suit your 5e tastes, you could always try the Celestial Pact Warlock instead, who gets a healing touch and a bunch of "kill it with fire/light" spells and abilities.