Short answers, pulled from the story.
A magic item in Dungeons & Dragons is any object imbued with magical powers. Magic items may act on their own or serve as tools for the character who possesses them, and they have appeared in every edition of the game since the original edition in 1974.
The Deck of Many Things was introduced in 1975's Supplement I: Greyhawk, created by Rob Kuntz and Gary Gygax. The original deck comprised 22 cards, each capable of producing immediate and powerful magical effects when drawn.
Item Rarity is a system introduced in the 5th edition Dungeon Master's Guide that rates magic items as Common, Uncommon, Rare, Very Rare, or Legendary, denoting how frequently each item is expected to appear in the game. Artifacts form a sixth category beyond Legendary, representing items of which only one exists in the game world.
Attunement is a process in 5th edition Dungeons & Dragons by which a character forms a magical bond with an item to access its properties. The standard 5th edition rules limit a character to three attuned items at any one time.
Several iconic D&D magic items were drawn from existing literature. Ioun stones came from Jack Vance's Dying Earth series, the Hand and Eye of Vecna were inspired by Michael Moorcock's Corum novels, the Vorpal Sword derives from Lewis Carroll's poem "Jabberwocky," and the Carpet of Flying traces back to Persian mythology popularized through 1001 Arabian Nights.
Encyclopedia Magica is a four-volume series published beginning in 1994 that lists all magical items published across two decades of TSR products. The four volumes together total more than 1,500 pages, with each volume containing over 1,000 magic items.