Appium is an open source test automation tool that allows users to easily write functional tests for iOS and Android mobile apps. It supports testing in multiple languages including Python, Ruby, Node.js, Objective-C, and Java. Appium can be used with JUnit or TestNG testing frameworks. To start using Appium, users need to download the sample projects, set up Appium in their environment, and run the tests. The test automation process involves creating a RemoteWebDriver instance with desired capabilities, specifying the app location, and running the tests. Tests can also be written to be run against Sauce Labs by updating the configuration to reference a zip file containing the app and pointing to ondemand.saucelabs.com. The tests themselves are similar to regular Selenium tests, allowing users to find elements, click buttons, and verify values. Appium provides sample Java tests for running tests against a simple TestApp, Sauce Labs, and the UICatalog sample application.