A binary tree is a hierarchical data structure composed of nodes, where each node has at most two children, referred to as the left child and the right child. The topmost node in a binary tree is called the root. Each node in the tree contains some data and may have a reference (link) to its left and/or right child nodes.
The structure of a binary tree is such that the values of nodes in the left subtree are less than or equal to the value of the node, and the values of nodes in the right subtree are greater than the value of the node. This property is not mandatory for all binary trees, as they can be unsorted, unlike Binary Search Trees (BSTs) where this ordering is crucial.