Let's look into the basic idea behind the use of stacks and queues using an analogy. Let's assume that we have a table where we put our incoming mail from our postal service, for example, Canada Mail. We stack it until we get some time to open and look at the mail, one by one. There are two possible ways of doing this:

• We put the letter in a stack and whenever we get a new letter, we put it on the top of the stack. When we want to read a letter, we start with the one that is on top. This is what we call a stack. Note that the latest letter to arrive will be on the top and will be processed first. Picking up a letter from the top of the list is called a pop operation. Whenever a new letter arrives, putting it on the top is called push operation. If we end up having a sizable stack and lots of letters are continuously arriving, there is a chance that we never get a chance to reach a very important letter waiting for us at the lower end of the stack.
• We put the letter in pile, but we want to handle the oldest letter first: each time we want to look at one or more letters, we take care to handle the oldest one first. This is what we call a queue. Adding a letter to the pile is called an enqueue operation. Removing the letter from the pile is called dequeue operation.