API, or Application Programming Interface, is a third-party library supplied by organisations to allow developers to use their services on websites. Other APIs allow data to be passed to the organisation’s servers to add to an account of the website.
Google, Facebook, and Twitter allow developers to use APIs to include their services on websites.
Google have a wide range of APIs available including YouTube, Maps, GeoLocation, Advertising, Google+, and Gmail, just to name a few.
Facebook have APIs for login using oAuth, sharing posts, and Messenger, while Twitter have an API for loading Tweets, posting Tweets, and retrieving the number of Tweets, Followers, Following, and many more.
The main API I use is the Google Maps API to display a map on a website, with specific co-ordinates of the location.