APIs enable distributed software to communicate over a network by exchanging messages.
REST is a popular style of APIs used today thanks to its simplicity and natural tendency to work well on the web.
In REST (or REST-like) APIs, servers and client applications exchange HTTP messages such as GET, POST and DELETE. REST APIs are commonly used as interfaces between mobile or web applications and their backends.
Increasingly, APIs are being considered valuable products themselves. They allow organizations to extend the reach of their data and services beyond internal boundaries. This enables them to tap into the ever growing ecosystem of third party developers and partners, from which enormous wealth and creativity can be found.