When you visit a friend’s Facebook profile, it’s designed for a person to look at. Nice fonts, pretty images, plenty of links, lots of eye candy.
Computers are pretty bad at clicking around on websites and looking at images, though. As a result, they get a special way to interact with the site: an API.
An API lets computers and web sites talk to each other without all those links and styles and pretty images getting in the way. Think of it as a version of the site stripped of everything but the most important pieces of information.
If you wanted to invite all of your Facebook friends to play Farmville with you, it’d be a pain to email them each individually. Instead, you can just ask Farmville to do it for you. How’s it get all their info? Farmville just uses Facebook’s API!
Every time you take a picture on Instagram or make a new post on Tumblr, you can automatically post it to your Twitter feed. How’s that magic happen? Instagram and Tumblr just use Twitter’s API!
So next time you log into Instagram or Farmville or some other new web app using Twitter or Facebook, you’ll know that APIs are hard at working behind the scenes helping sites communicate.