This document discusses double-ended queues or deques, which are data structures that allow elements to be added or removed from either end. It defines deques as linear data structures that act like both stacks and queues by supporting insertions and deletions from both ends. The document outlines different types of deques based on insertion and removal restrictions and describes common deque operations like insert_front, which pushes an element to the front of the deque by decrementing the front pointer and inserting the element at that index.