# 2. Prerequisite: An Introduction to the Tree Data Structure
Before we can understand the Heap, we must first be familiar with its underlying structure: the Tree. In computer science, a **tree** is a widely used data structure that simulates a hierarchical structure, with a set of connected nodes.
#### **Core Components**
Every tree is composed of a few fundamental components:
- **Node:** The primary entity in a tree that contains data. It is also known as a "vertex".
- **Edge:** A connection or link between two nodes.
- **Root:** The topmost node in a tree. It is the only node that does not have a parent.
Consider the tree structure below:
- Nodes: {A, B, C, D, E}
- Edges: {(A-B), (A-C), (B-D), (B-E)}
- Root: Node A
#### **Hierarchical Terminology**
The relationships between nodes in a tree are described using terminology borrowed from family trees:
- **Parent:** A node that is directly above another node and connected to it. In our example, A is the parent of B and C.
- **Child:** A node that is directly below another node and connected to it. In our example, D and E are children of B.
- **Leaf:** A node that has no children. In our example, C, D, and E are leaf nodes.
#### **Structure and Depth**
We can measure the structure of a tree in several ways:
- **Level:** The level of a node is defined by its distance from the root. The root is at **Level 0**. Its children are at Level 1, their children are at Level 2, and so on.
- **Depth/Height:** The height of a tree is the number of edges on the longest path from the root to a leaf. Our example tree has a height of 2.
#### **Focus on the Binary Tree**
While a node in a tree can have any number of children, our focus for understanding heaps will be on a specific type: the **Binary Tree**.
A **Binary Tree** is a tree data structure in which each node has **at most two children**, which are referred to as the left child and the right child.
The reason we focus on Binary Trees is that the **Heap** data structure, which is the engine of Heap Sort, is a specialized type of Binary Tree. Mastering this concept is a fundamental step toward understanding heaps.