Kanban in its simplest state is a signal linking two or more processes. There are many different types of kanban signal and many uses. There are not many true, working systems outside of Toyota.

Usually, a card or sheet is used to authorise manufacture or movement of an item. Normally, the kanban signal ensures that manufacture and movement of parts and material take place only as required by a downstream operation. It becomes part of a pull system of manufacture.
The kanban for a supplier is different in design but operates as part of the pull system to ensure that replenishment of stock only occurs when the customer pulls an item and that it is supplied only in the quantity required and in the time required.

In an ideal system the kanban system should be a unit of one. The number of kanban cards in the system is calculated taking into account a number of factors such as lead time cycle time. When a company has a culture of continuous improvement it would be normal to expect the kanban number to be reduced and, in the case of intermediary stock, eliminated.

Beware, fake kanbans are prolific because Shigeo Shingo stated that there are 12 prerequisites to satisfy before a kanban system will work-

1. Continuous production
2. Stable production volume
3. Streamlined and stable production processes
4. Invariable quality
5. Thorough operational understanding
6. Defects addressed immediately
7. Establishment of clear defect criteria
8. 5S
9. Scheduling is managed by kanban
10. Swift and thorough execution of all decisions
11. Short set up time
12. Preventative maintenance and rapid machine repair