Equip Spell Cards are placed in the Spell & Trap Card Zone of the activating player; other Equip Cards are placed in the Spell & Trap Card Zone of the player whose effect caused the card to be treated as an Equip Card. Equip Cards can be equipped to monsters on either player's side of the field. If a Monster Card is treated as an Equip Card, it is no longer treated as a monster; if a Spell or Trap Card is treated as an Equip Card, it is still treated as a Spell or Trap Card, respectively.
A monster can be equipped with any number of Equip Cards, as long as there are Spell & Trap Card Zones for these Equip Cards.
Cards that equip themselves target a monster upon activation, and are then equipped to the monster they targeted upon resolution. Equip Cards constantly target the monster they are equipped to; even while negated, Equip Cards remain equipped and continue to target the monster. If the equipped monster is no longer face-up on the field, or is no longer a valid target, all cards equipped to it are immediately destroyed.
In the anime, monsters will sometimes hold items they've been equipped with, for example in the waking of the dragons arc, The Fiend Megacyber picks up the Time Magic Hammer.