A pile, or stack, is a data structure that follows the Last In, First Out (LIFO) principle. This means that the last item added to the stack is the first one to be removed. Stacks are commonly used in programming for tasks such as managing function calls, undo mechanisms in applications, and parsing expressions. In a stack, operations are typically limited to two main actions: push, which adds an item to the top, and pop, which removes the item from the top. Stacks can be implemented using arrays or linked lists, making them versatile for various applications in computer science.