Which data structure uses the concept of 'first-in, first-out'?

The concept of 'first-in, first-out' (FIFO) is a defining characteristic of a queue. In a queue, the first element that is added to the structure will be the first one to be removed. This behavior is analogous to a line of people waiting for service: the person who arrives first is served first.

In practical applications, queues are used in scenarios such as task scheduling in operating systems, handling requests in web servers, or implementing breadth-first search in graph algorithms. The operations on a queue mainly include enqueue (to add an item to the back) and dequeue (to remove the item from the front), which reinforces the FIFO methodology.

Other data structures mentioned operate differently. A tree organizes data hierarchically, a stack follows a 'last-in, first-out' (LIFO) principle where the most recently added item is the first to be removed, and a hash table provides key-value pairs for fast access, rather than any specific order of processing elements. Thus, the queue is uniquely suited to the FIFO characteristic.