Guardian writes about the third era – after static web pages and dynamic web pages.
The programmable web is different for two main reasons. First, instead of going to look at a web page, you can get a computer to extract the information for you. Second, you don’t have to view that information in a browser: you could use it in a different application, or on a different device, such as a mobile phone. When websites make information available in this way, they are called web services.
The important point is that you didn’t need to go to the web to get useful information from a website: the web is no longer just about “eyeballs,” it is also about computers talking directly to computers on your behalf. The corollary is that someone has to specify how all these applications talk to one another, and provide an applications programming interface (API). This tells you how to frame a request in order to get the right response.