The Heart Container (ハートのうつわ, hāto no utsuwa), known simply as Heart in Smash 64, is an item that will reduce the damage of the character who picks it up. Its operation has remained functionally identical, but the nuances of its healing have changed in some of the games.
In standard modes
The Heart Container never spawns on the ground; it will always spawn in the air and slowly fall down towards the stage, though it can be consumed while it is in the air.
Upon consumption, the Heart Container recovers an amount of health dependent on the game. In Super Smash Bros., it will heal the player's health down to 0%; in the later games, it can only heal 100% worth of damage.
In the original game, the Heart Container can "absorb" damage as the player's health decreases; as a result, being hit by an attack that does 10% damage will not inflict the 10% damage while recovering damage. This is not the case in later games.
In Super Smash Bros. and Melee, consuming at least two or three Heart Containers in a match, respectively, gives the player the Heartthrob bonus, worth 17,000 points in Smash 64 and 2,800 points in Melee. As the Maxim Tomato and Food in Melee are considered Food items instead of recovery items, consuming three Heart Containers is the only way to attain the Wimp bonus, worth 1,500 points; as a result, the Wimp bonus always overlaps with Heartthrob.
In All-Star Mode
In the All-Star Mode in Melee and Brawl, as well as Brawl's Boss Battles mode, the player is given three Heart Containers in the All-Star Rest Area that can heal all of their damage instead of the standard 100%. In the two-player, cooperative modes in Brawl, the game instead gives players five Heart Containers; in Smash 4's All-Star Mode, the player receives only one Heart Container initially and gets two if playing after unlocking all characters, but they also receive a Maxim Tomato and Fairy Bottle. Completing All-Star Mode in Melee without using a Heart Container or Continuing gives the player the Melee Master bonus, worth 100,000 points.
A bug with Melee's teleporter causes the Heart Container to not properly heal the player if they enter it while recovering damage; this bug was fixed in later games.
As a sticker
A sticker depicting a Piece of a Heart Container can be attained in Brawl. Usable only by Link, Zelda, Ganondorf and Toon Link, the sticker causes Heart Containers to heal 150% damage instead of only 100% damage.
The Heart Container made its first appearance in The Legend of Zelda. At the beginning of The Legend of Zelda, Link starts off with a maximum health capacity of three hearts. When Link finds a Heart Container, it gives him another heart of capacity, allowing him to sustain more damage before dying. Starting in The Legend of Zelda: Link's Awakening Heart Containers would also replenish Link's health to full upon collection. In Super Smash Bros. and Melee, the Heart Container's design is based around Ocarina of Time's Heart Containers. Its appearance changed in Brawl to the design in The Legend of Zelda: Twilight Princess, like most Zelda content.  The design of the Heart Container changed once again in Super Smash Bros. 4 to match its appearance in The Legend of Zelda: Skyward Sword, giving it a slightly thicker and more detailed aesthetic.
Heart Containers drift toward the ground slowly and replenish 100 percentage points of health. In The Legend of Zelda, these items beefed up Link's life meter; in most cases, Link found them in the possession of bosses or hidden in secret areas. Heart Containers are often scattered across the land in several fragments.
The best recovery item! Grab this and heal 100 percentage points of damage. It falls slowly, so it's best to grab it in midair. Damage recovery won't happen instantly, though, so try not to get launched while your damage is counting down. In the Legend of Zelda series, Heart Containers raise the player's maximum health. Bosses always have one in their possession.