A Soul Jar is any object that has been infused with a portion of the soul of some intelligent being. The method for creating them is detailed in the Black Book.
Background
Soul Jars were first created by Zebras as a method of obtaining immortality. The idea was that if a portion of the soul were captured within an object, and that object were made indestructible, the bearer of the original soul could never die. Whether or not immortality was achieved, the Soul Jar itself has been proven to be immune to any damage, making the spell at least partially successful.
It is possible that the underlying mechanics of creating a Soul Jar were used in the inspiration of the Megaspell, namely in the idea of "hanging" spells.
It stands to reason that if Zebras designed the original soul jar, and if Ponies were able to successfully replicate them, that other intelligent beings such as Diamond Dogs (now Hellhounds), Griffins, Buffalo and Dragons could be soul "donors". Aside from the magic of Dragon fire, these other species have not demonstrated any aptitude for magic and would likely not be able to create a Soul Jar without the help of a Zebra or Unicorn Pony.
Known Soul Jars
The Black Book - The Black Book itself, whilst holding the details to making a Soul Jar is itself a Soul Jar and invulnerable to damage.
Ministry Mare Statuettes - The Statuettes are Soul Jars, created from the soul of Rarity
Crusader Maneframe - Celestia turned the Crusader Maneframe, located within the S.P.P Tower into a Soul Jar.
In side Stories
Octavia's Contrabass (Project Horizons) - Octavia's Contrabass was turned into a soul jar when a piece of her soul was placed into her instrument.
Phoenix Talisman (Project Horizons) - Rampage has a Talisman within her that is impervious to damage, on account of being a Soul Jar. It continually revives her no matter how thoroughly she is killed or destroyed. It also has multiple souls contained in it.
Zyon's Sword (Broken Steel) - Zyon's family sword is a soul jar and an heirloom that has been passed down his family's line for generations. It was inherited from his Zebra grandparents.
The Ditzy Doo Chronicles - Snips creates a sword and later makes it into a soul jar, knowing the spell from when he had done it to Rarity. The sword was impervious to damage, and other unicorns couldn't hold it with their magic, at least not without feeling extreme pain.