A tree is data arranged in a hierarchy: one root at the top, each node with some children below it, and no loops. They're everywhere: folders and files, a company's org chart, the structure of a web page, a family tree, the moves in a game.

Term	Means
root	the top node, with no parent
child, parent	a node directly below or above another
leaf	a node with no children
subtree	a node together with everything below it
height	the number of levels on the longest path from the root down

In code, a tree is usually a node class holding a value and a list of children, which are nodes themselves:

class Node:
    def __init__(self, value, children=None):
        self.value = value
        self.children = children or []

tree = Node("CEO", [
    Node("CTO", [Node("Dev 1"), Node("Dev 2")]),
    Node("CFO", [Node("Accountant")]),
])

def show(node, depth=0):
    print("  " * depth + node.value)
    for child in node.children:
        show(child, depth + 1)

show(tree)

Notice how naturally recursion fits: to show a tree, show the root, then show each child's subtree.