A queue is an ordered collection of items where the addition of new items happens at one end, called the rear, and the removal of existing items occurs at the other end, commonly called the front. As an element enters the queue it starts at the rear and makes its way toward the front, waiting until that time when it is the next element to be removed.
The most recently added item in the queue must wait at the end of the collection. The item that has been in the collection the longest is at the front. This ordering principle is sometimes called FIFO, first in, first out. It is also known as first come, first served.
Queues maintain a FIFO ordering property.