PUSH vs POP. A stack is a data structure that is used in programming. There are two basic operations that can be performed on a stack to modify its contents, which are called PUSH and POP. The main difference between PUSH and POP is what they do with the stack. PUSH is used when you want to add more entries to a stack while POP is used to remove entries from it. A stack is so named because it places the individual data entries just like a stack of books.