Notes

The law states that average queue time will equal the average queue size divided by the average processing rate.

It also states that the longterm average number $L$ of work in a system equals the longterm average effective arrival rate $\lambda$ multiplied by the average time $W$ that the work stays in the system.

MIT professor John D. C. Little proved it in 1961.

This formula is robust; it applies to virtually all queues disciplines, arrival rates, and departure processes ($W_Q$ is the queue time for an average job, $L_Q$ number of jobs in a queue, $\lambda$ average processing rate, $W_S$ is the system time for an average job, $L_S$ number of jobs in the system).
 You can use the formula for the whole system instead of just for a queue. This use is helpful when you have trouble distinguishing which items are in the queue and which ones are in service.
Examples
Assume customers arrive at a rate of 10 per hour in a store and stay 0.5 hours on average. The average number of customers in the store is $L = 10 * 0.5 = 5$.