Stack is the data structure that follows the Last-In-First-Out (LIFO) principle.

- Stack:
A stack is an abstract data type that represents a collection of elements. It follows the LIFO principle, which means that the last element added to the stack will be the first one to be removed. The operations performed on a stack are push (to add an element to the top of the stack) and pop (to remove the topmost element from the stack).

- Stack Visualization:
A stack can be visualized as a vertical stack of elements, where the topmost element is the one accessible for operations. When a new element is added, it is placed on top of the stack, and when an element is removed, the topmost element is removed.

- Example:
Consider the following example of a stack:

Initial stack: Empty

Operations:
1. Push element A
Stack: A
2. Push element B
Stack: B -> A
3. Push element C
Stack: C -> B -> A
4. Pop
Stack: B -> A
5. Push element D
Stack: D -> B -> A
6. Pop
Stack: B -> A

In the above example, the elements are added to the top of the stack using the push operation, and the topmost element is removed using the pop operation. The last element added (D) is the first one to be removed.

- Applications of Stack:
Stacks are widely used in various applications, including:
- Function call stack in programming languages
- Undo/redo functionality in text editors
- Expression evaluation and postfix conversion
- Backtracking algorithms
- Depth-first search in graphs

- Conclusion:
In conclusion, a stack is the data structure that follows the LIFO (Last-In-First-Out) principle. It is represented as a vertical stack of elements, where the last element added is the first one to be removed. Stacks are used in many applications and are an essential concept in computer science and software development.