The Power of APIs
An API is a software-to-software interface, not a user interface. Servers can talks to each other without any user knowledge or intervention. When you shop online and enter your credit card information, the web site uses an API to send your credit card information to a remote application that verifies whether your information is correct. Once payment is confirmed, the remote application sends a response back to the web site saying it's OK to deliver the item. The user usually never notice when software functions are handed from one application to another.