Stack

LIFO

You only ever touch the top element. push, pop, and peek are all O(1). Backed by an array (append/pop) or a linked list (insert/remove at head).

Stack of plates

You add and take plates from the top. The first plate you put down is the last one you pick up.

Balanced parentheses

Push openings, pop and match on closings. Empty stack at the end means balanced.

def is_balanced(s):
    pairs = {')': '(', ']': '[', '}': '{'}
    st = []
    for c in s:
        if c in '([{':
            st.append(c)
        elif not st or st.pop() != pairs[c]:
            return False
    return not st

Output:

is_balanced("({[]})") -> True
is_balanced("(]")     -> False

Where it hides

Function call stack, undo/redo, browser history, expression evaluation, DFS, and "next greater element" (monotonic stack).