A stack is one of the most fundamental data structures in computer science. It operates on a simple principle known as LIFO—Last In, First Out. This means that the last item added to the stack is the first one to be removed. Think of a stack like a stack of plates: you add new plates on top, and when you need a plate, you take the one at the top first. The two primary operations for a stack are 'push' (to add an item) and 'pop' (to remove an item). Stacks are used in various real-world applications. For instance, when a function is called in programming, its execution context is pushed onto the call stack. Once the function completes, its context is popped from the stack. This is why recursion, where functions call themselves, is often managed with a stack. Another common use of stacks is in undo features of software. Every action a user performs is pushed onto a stack, and when the user presses 'undo,' the most recent action is popped off the stack and undone. Stacks are also used in depth-first search (DFS) algorithms, where nodes or vertices are explored deeply before backtracking, which mirrors the stack’s LIFO principle. Despite being a simple structure, stacks are incredibly powerful and versatile, often serving as the backbone for many more complex algorithms. Their efficiency comes from the fact that all operations (push and pop) can be performed in constant time O(1), making them ideal for scenarios where order matters and only the most recent item needs to be accessed.