APIs detail data interaction between applications. On the web, you can use APIs to request data from or send data to any of the platforms that support them. Consequently, you can use APIs to extend services these platforms provide or build custom solutions on them, whether it is to download all pictures you've liked on Instagram, add the birthdays of your Facebook friends to your Google Calendar or listen to the Twitter stream for tweets containing the words 'discount' or 'coupon'.
This book introduces REST APIs in a simple way. It shows how web applications make data available via their respective APIs and how easy it is to use them.