A keycard is a small credit card-sized device that opens doors. Sometimes a keycard uses a magnetic strip (like a credit card) or a barcode, but most often use RFID (radio frequency identification). For the paranoid, keycards are convenient because it means that they can keep track of who opened what door when (because every keycard transmits a different ID -- it's like everyone has a different key, but they all open the same lock.) Another advantage of keycards is that revoking access is a simple matter of telling the computer not to allow certain ID's through.
Hotels have started to use keycards too. They use some sort of optical system. The guest is given a thin cardboard card with holes punched in them. The door unlocks for a certain (resetable) pattern of holes. The nice thing about this system (from the hotel's point-of-view) is that the keycards are cheap and disposable.
The major disadvantage of a keycard system is that they need electricity to work. That's why almost all doors that are controlled by keycards also have a regular keyed lock as well.