Stack

Stack - [a, b, c, d]

Start:
  node4 -> node3 -> node2 -> node1

  Stack - no idea how many items are in it, only knows the top item
    @data = node4

Next - Add a new node to the stack - "push"
  push(value)
    Use that value to create a new node (New Node)
    Take our new node and add a reference to the last existing node - represented by @data
    @data = New Node, which essentially becomes node5

Now
  Stack
    @data = node5


POP
Now
  node5 -> node4 -> node3 -> node2 -> node1
  
  pop
    return the value from node5
    update the reference of node5 to nil