A kanban is a signaling device that gives authorization and instructions for the production or withdrawal (conveyance) of items in a pull system. The term is Japanese for “sign” or “signboard.”
Kanban cards are the best-known and most common example of these signals. They often are slips of card stock, sometimes protected in clear vinyl envelopes, stating information such as part name, part number, external supplier or internal supplying process, pack-out quantity, storage address, and consuming process address. A barcode may be printed on the card for tracking or automatic invoicing.
Besides cards, kanban can be triangular metal plates, colored balls, electronic signals, or any other device that can convey the needed information while preventing the introduction of erroneous instructions.
Whatever the form, kanban have two functions in a production operation: They instruct processes to make products, and they instruct material handlers to move products. The former use is called production kanban, while the latter use is known as withdrawal kanban.
Source: “Lean Lexicon: a graphical glossary for Lean Thinkers Fourth Edition,” Lean Enterprise Institute.