A complete binary tree must have each level be completely occupied except the last level and have no gaps in the last level (so each node in the last level is as far to the left as it can be). 

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A tree is an abstract data structure that represents a hierarchical tree. A tree contains a root which is connected to subtrees through linked nodes. The tree data structure is inherently recursive, since a node's subtree is also a tree in and of itself.
A node that has a subtree is the "parent" node of the subtree. The nodes within the subtree of the parent node are called "child" nodes. Child nodes that have the same parent are known as "sibling" nodes.

There are many types of tree data structures. A few of them include:
- Binary Tree (Binary Search Tree)
     1. AVL Tree
     2. Red-Black Tree
     3. Splay Tree
     4. Treap
- B-Tree
- N-ary Tree

Tree

https://web.cecs.pdx.edu/~sheard/course/Cs163/Doc/FullvsComplete.html#:~:text=A%20full%20binary%20tree%20(sometimes,as%20far%20left%20as%20possible.

Full and Complete Binary Tree

A binary search tree is a specific type of tree that orders its subtrees based on the value of the node. For every node in the tree, the left sub tree must have a value less than the node (or parent) and the right sub tree must have a value greater than or equal to the node.

Binary Search Tree

A B - tree is a self balancing search tree. They are used to lower the amount of times that a disk is accessed - which is how the data from the tree is read. As a result, the number of keys (or values) in a given node of the tree is maximized to lower the height of a B - Tree.

B - Trees

A N - Ary Tree is a tree data structure where every node does not have more than N children. For example, every node in a 3 - Ary tree has either 0, 1, 2, or 3 children nodes. 

N - Ary Tree

Complete Binary Tree

A full binary tree is a binary tree where each node must have either zero or two children. In effect, each node has two children except for the leaf nodes.

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