Equipment is an artifact subtype that has the "equip" keyword ability. By paying the equip cost, it attaches to a creature you control. If that creature leaves play, the Equipment becomes unattached and remains in play. Paying the equip cost also allows the Equipment to be moved from one creature to another.
Equipment was created as an attempt to improve on the concept of creature enchantments — specifically, the inherent card disadvantage of attaching Auras to a creature. First appearing throughout Mirrodin block, that block's heavy artifact theme presented an opportunity to represent a creature's magical weapons and devices as artifacts instead of enchantments, and introduce new rules for using them. Equipment was introduced to core sets in 9th Edition.
Alara Reborn introduced the first colored equipment.
From the Comprehensive Rules
(Battle for Zendikar
(September 26, 2015
301.5. Some artifacts have the subtype “Equipment.” An Equipment can be attached to a creature. It can’t legally be attached to an object that isn’t a creature.
- 301.5a The creature an Equipment is attached to is called the “equipped creature.” The Equipment is attached to, or “equips,” that creature.
- 301.5b An Equipment is cast and enters the battlefield just like any other artifact. An Equipment doesn’t enter the battlefield attached to a creature. The equip keyword ability attaches the Equipment to a creature you control (see rule 702.6, “Equip”). Control of the creature matters only when the equip ability is activated and when it resolves. Spells and other abilities may also attach an Equipment to a creature. If an effect attempts to attach an Equipment to an object that can’t be equipped by it, the Equipment doesn’t move.
- 301.5c An Equipment that’s also a creature can’t equip a creature. An Equipment that loses the subtype “Equipment” can’t equip a creature. An Equipment can’t equip itself. An Equipment that equips an illegal or nonexistent permanent becomes unattached from that permanent but remains on the battlefield. (This is a state-based action. See rule 704.) An Equipment can’t equip more than one creature. If a spell or ability would cause an Equipment to equip more than one creature, the Equipment’s controller chooses which creature it equips.
- 301.5d An Equipment’s controller is separate from the equipped creature’s controller; the two need not be the same. Changing control of the creature doesn’t change control of the Equipment, and vice versa. Only the Equipment’s controller can activate its abilities. However, if the Equipment grants an ability to the equipped creature (with “gains” or “has”), the equipped creature’s controller is the only one who can activate that ability.
- 301.5e An ability of a permanent that refers to the “equipped creature” refers to whatever creature that permanent is attached to, even if the permanent with the ability isn’t an Equipment.
- Paul Barclay. (September 2, 2003.) "Rules for Equipment", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Mark Rosewater. (August 22, 2005.) "Equipment To Be", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Aaron Forsythe. (August 26, 2005.) "The Power That is Equipment", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Magic Arcana. (February 18, 2004.) "The Flavor of Equipment", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Magic Arcana. (August 22, 2005.) "The Flavor of Equipment II", Daily MTG, magicthegathering.com, Wizards of the Coast.