Items (customers, patients, cars) enter a system (check-out, hospital, repair shop) for service.
They may have to wait in-line, in the queue, to be serviced.
When they finally receive service, this also takes time.
When they are done, they depart the system, this may also take time.
Queuing time and service time, (and departure time) are often considered together.
We are interested in quantities such as how many items are in a system at a time \(t\), average time in the system, and the stability of the queue.