What is the Queue ADT, and what are its operations?

Question

Accepted Answer

A queue stores objects in first-in, first-out (FIFO) order: enqueue(o) adds at the rear, dequeue() removes and returns the element at the front. Helpers are first(), size() and isEmpty(); dequeue() and first() on an empty queue return null.
Think of a queue at a counter: newcomers join at the back, and the person served next is the one who has waited longest.

Operation
What it does

enqueue(o)
insert o at the rear

dequeue()
remove and return the element at the front

first()
return the front element without removing it

size()
number of stored elements

isEmpty()
whether any elements are stored

The difference from a stack is where elements leave. A stack adds and removes at the same end, so the newest element comes out first. A queue adds at one end and removes at the other, so the oldest comes out first.
Go deeper:

Queue (abstract data type) — operations and the linked-list and circular-array implementations.
 Queue visualisation, linked-list (USF) — enqueue at the tail, dequeue at the head.

Operation	What it does
`enqueue(o)`	insert `o` at the rear
`dequeue()`	remove and return the element at the front
`first()`	return the front element without removing it
`size()`	number of stored elements
`isEmpty()`	whether any elements are stored