When two computer programs talk to each other, they use an interface and it is called Application Programming Interface, abbreviated as API. When we want to buy vegetables, we use the name of the vegetable and each vegetable has a price per unit and a quantity expressed in units. If a computer program is called “Vegetable Buyer” and another program is called a “Vegetable Seller” and these two programs want to do a transaction then we use the Application Programming Interface.