The Appium server is used to interact with different platforms such as iOS and Android. It creates a session to interact with mobile apps, which are not supported on any platform. It is an HTTP server written in Node.js and uses the same concept as the Selenium Server, which identifies the HTTP requests from the client libraries and sends these requests to the appropriate platform. To start the Appium server, users need to download the source or install it directly from npm. Appium also provides the GUI version of the server. You can download it from the official Appium site, http://appium.io. In the next chapter, we will discuss the GUI version in more detail.

One of the biggest advantages of Appium is because it is simply a REST API at its core, the code you use to interact with it is written in a number of languages such as Java, C#, Ruby, Python, and others. Appium extends the WebDriver client libraries and adds the extra commands in it to work with mobile devices. It provides client libraries that support Appium extensions to the WebDriver protocol. Because of these extensions to the protocol, it is important to use Appium-specific client libraries to write automation tests or procedures, instead of generic WebDriver client libraries.

Appium added some interesting functionality for working closely with mobile devices, such as multitouch gestures and screen orientation. We will see the practical implementation of these functionalities later.