One of the three main types of items commonly found in Stages in Treasure Trove, alongside Treasures and Food, the Jars are restorative pickup items used to replenish the protagonists's ability to use secondary items. Enemies are insured to, eventually, drop jars. Any jar that was not originally out in the open when first found (for example being dropped by enemies), will eventually blink out of existence if not collected quickly. Any previously collected jar who had a fixed location (out in the open, found in Dirt Blocks,...), will respawn upon death where it was originally found. Their appearance and functions vary slightly with each campaign:
Magic Jars[]
In Shovel of Hope, Shovel Knight can pick up Magic Jars to restore specific values of his numerically expressed Magic Capacity, which is used to activate Relics. Small Jars restore 5 magic whist the large ones restore 30 magic. Magic Jars are found commonly throughout the game by defeating enemies, breaking blocks or at specific spots in stages. The use of an Ichor of Fortune will magnetize any jar in range towards Shovel Knight.
Wearing the Conjurer's Coat insures that enemies drop Jars more frequently, while wearing the Armor of Chaos will transform the magic jars into blue orbs that home in on Shovel Knight.
The Magic Capacity can be further increased by buying Magic Drinks from the Magicist.
During Memories, Donovan also benefits of a personal Magic Capacity and as such can find and pick up Magic Jars.
Power Jars[]
In Plague of Shadows, Plague Knight can pick up Power Jars to restore part of his Power Bar, which is used to cast Arcana. They appear throughout the game in the same way Magic Jars do. Small Jars restore half a portion of Plague Knight's Alchemy Power while a large one restore two portions. Unlike Magic, Power naturally regenerates over time. Using the Treasure Trappings will magnetize any jar in range towards Plague Knight.
The Power Bar can be further extended by buying Alchemy Power Upgrades from the Magicist.
Darkness Jars[]
In Specter of Torment, Specter Knight can find Darkness Jars to restore a portion of his Darkness Capacity, which is a meter used to power Curios. They appear throughout the game but they also drop and home in towards Specter Knight in the form of little tufts of purple flames when he defeats enemies or sufficiently hurts the more bulky ones (similarly to the red orbs generated by the use of the Armor of Chaos). For the darkness flames to appear the enemy must have been damaged at least once by Specter Knight as, if the enemy is defeated without any direct intervention from him (for example falling down a pit on it's own accord) no flames will appear.
Small Jars and flames restore half a portion of Specter Knight's Darkness while large Jars restore three portions. Candles with purple fire, usually found in the Tower of Fate, generate an endless source of Darkness flames.
The Darkness Capacity can be further extended by finding Darkness Wisps hidden within the Explodatorium, the Clockwork Tower and Lich Yard or by buying them from Missy, if failed to be located the first time.
In New Game Plus, Specter Knight's Will and Darkness are fused in one ever-depleting meter, and as such Darkness Jars and flames act at the same time also like a food source, and passing by a Checkpoint for the first time will completely refill Specter Knight's fused "Darkness+Will" gauge.
Vigor Vials[]
In King of Cards, King Knight can find Vigor Vials to restore specific values of his numerically expressed Vigor Capacity, which is used to activate Heirlooms. Small Vials restore 10 vigor whist the large ones restore 30 vigor. Vigor Vials are found commonly throughout the game by defeating enemies, breaking blocks or at specific spots in stages. Unlike other magnet like abilities, the use of Hengineer's Wealth Whirl upgrade will not magnetize any jar in range towards King Knight.
In certain stages, such as Heirloom stages or Challenges, statues depicting the Vicar of Vigor will generate an endless source of little tufts of blue and green flames-like orbs that restore Vigor.
Furthermore, wearing the Vestments of Vigor insures that vigor flames are released when King Knight defeats enemies or sufficiently hurts the more bulky ones (similarly to the Darkness flames generated for Specter Knight or the red orbs generated by the use of Shovel Knight's Armor of Chaos). For the vigor flames to appear the enemy must have been damaged at least once by King Knight as, if the enemy is defeated without any direct intervention from him (for example falling down a pit on its own accord) no flames will appear.
The Vigor Capacity can be further increased by buying Vigor Cream Pie from Mom with Merit Medals.
In New Game Plus, due to the Heirlooms Vigor cost being switched out with an cost in Gold, the Vigor Vials instead act as Treasures, with a small Vial being worth, without counting any multiplier, 
40 in and a big one being worth 100.