A stack is a linear data structure that follows the LIFO (Last In First Out) principle, meaning that the last element inserted into the stack is the first one to be removed). The stack data structure is used in many applications, such as expression evaluation and conversion, backtracking, and parentheses checking.

A stack can be implemented using contiguous memory, such as an array, or non-contiguous memory, such as a linked list). The stack data structure allows all data operations at one end only, and at any given time, only the top element of the stack can be accessed.

The basic operations on a stack include push, which adds an element to the top of the stack, and pop, which removes the top element from the stack). Other operations include peek, which returns the top element of the stack without removing it, and isEmpty, which checks if the stack is empty.

In summary, a stack is a data structure that follows the LIFO principle and is used in many applications. It can be implemented using contiguous or non-contiguous memory, and its basic operations include push, pop, peek, and isEmpty.