What is an API? How different apps exchange data – Business Insider

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An API, or application programming interface, is software that acts as an intermediary between two other programs — or two components within a program — to exchange information. APIs are common types of computer code and form the foundation of our modern information architecture. 
APIs consist of two components, and both are routinely referred to as “the API,” which can be confusing:
APIs exist primarily to simplify the process of creating and maintaining software, in addition to extending and enhancing a program’s capabilities.
Consider a program like a word processor, for example. In the earliest days of PCs, if a word processor like Microsoft Word needed to print a document, the developers had to include code that allowed the software to communicate with every printer a user might possibly own.
Windows solved that problem by offering a library of printer drivers built into the operating system; each driver only needed to be written once, and programs simply use the printer API to access any printer. 
This is also an example of how APIs offer a layer of abstraction. The word processor doesn’t need to know how to print to a particular printer; it simply sends a print request to the printer API, and the API handles the how of printing.
APIs are not all alike, and the easiest way to differentiate APIs is by how they’re deployed. There are four main categories:

Because APIs are the glue that binds different programs together, they need to follow standard protocols so that any developers who use the API understand how to integrate it into their code. There are a handful of common specifications in use today.
Each one of these is a different way to standardize the way data is exchanged between programs, which is important since an API should be able to work regardless of how the program is written or even what language is used to code it.
These are the most common protocols used to develop API specifications today:
APIs are a core component of most modern software, so we are surrounded by them. Here are a few examples of APIs in common use: