A card reader is a data input device that reads data from a card shaped storage medium. The first were punched card readers which read the paper or cardboard punched cards that were used several decades of computing industry to store informations & programs for the systems. Modern card reader are electric devices that can read plastic cards embedded with either barcode, magnetic strip, computer chip or another storage medium.
The card reader on doors constantly emits a Radio frequency energy field. When a card crosses the field the power from the field energizes a copper wire inside the card which power the chip containing the card number and other unique data.