Queueing theory is a mathematical framework created to explain waiting lines.
You need some fundamental elements to define a queue:
An arrival statistics (how the queues grow);
The servicing rate (how the queue shrinks); and
The number of servers.
In the most common queue, both the intervals between successive arrivals and the servicing times are described by exponential distributions, and there is a single server. The corresponding model is the M/M/1 queue. Read more