A complete binary tree is a special type of binary tree where all levels of the tree are filled completely except the lowest level nodes, which are filled from left to right. In other words, all the nodes are as far left as possible in a complete binary tree. Some key features of a complete binary tree include:

• Root: The node in which no edge is coming from the parent.
• Child: The node having some incoming edge is called a child.

A complete binary tree is different from a full binary tree in that the last levels nodes are filled from left to right, and the last leaf element might not have a right sibling. In contrast, a full binary tree has every node with either two children or no children.

Complete binary trees can be efficiently represented using an array. They have several benefits, including faster insertion and deletion than linked lists and arrays, and a flexible way of holding and moving data.