Appium

Appium is an open-source tool for automating native, mobile web and hybrid applications on ios and Android platforms.

when you download the Appium app you are basically downloading the server. Appium server is written in Node.js and implements the selenium web driver. Appium allows one to use available Web Driver clients to fire your tests and your mobile app acts precisely  like a web app where the DOM is represented by view hierarchy. 

Appium  server basically exposes REST API which performs below actions.

1) Receive Connection from Client.

2) Listen Command.

3) Execute Command.

4) Respond back to the command execution status.

How it works :

1. Web Driver Script will send the Automation Commands in form of JSON via HTTP request to Appium Server ( Node.JS ).

2. Appium Server invokes Vendor Specific mechanism to execute these commands in device.

3. Device Sends back to the message to the Appium Server.

4. Appium Server logs the result in the console to web Driver and at Appium Server.