The last few years have seen the rise of Web APIs – services exposed over plain, general purpose web protocols (such as HTTP) rather than through a more formal service contract (like SOAP or WS*).  Exposing services this way makes it easier to integrate functionality with a broad variety of device and client platforms. Most large sites on the web now expose Web APIs (some examples: Facebook, Twitter, LinkedIn, Netflix, etc.), and the usage of them is accelerating even more in the years ahead as connected devices proliferate and users demand richer user experiences.

Additional Techie Info

REST (REpresentational State Transfer) is an architectural style and is often preferred over the more heavyweight SOAP (Simple Object Access Protocol) style because REST does not leverage as much bandwidth, which makes it a better fit for use over the Internet.

REST’S decoupled architecture, and lighter weight communications between producer and consumer make REST a popular building style for cloud-based APIs, such as those provided by Amazon, Microsoft, and Google. When Web services use REST architecture, they are called RESTful APIs (Application Programming Interfaces) or REST APIs. The REST style emphasizes that interactions between clients and services is enhanced by having a limited number of operations (verbs). Flexibility is provided by assigning resources (nouns) their own unique Universal Resource Identifiers (URIs).

Since each verb has a specific meaning (GET, POST, PUT and DELETE), REST avoids ambiguity. Instead of structuring the system with a single monolithic data set and a large block of code to get data in and out of it, you create a wide array of services, each of which has a public interface letting users retrieve data from it via the HTTP GET command, or change data using the HTTP POST command.