The computer world is based on using APIs. Zynga uses the Facebook APIs to embed its games on FaceBook. Any iPhone or iPad app uses the iOS API to let you move stuff by swiping the screen. WordPress is built on a series of APIs that lets just about anybody build a web site in minutes.
However, even the best of these APIs are hard to use because the documentation, supposedly written in English, is terrible. Most engineers can't write a single coherent sentence, never mind string together a paragraph. (Bernard Meisler at The Atlantic)