Cordova 3.x is a hybrid mobile app development framework that allows building apps that can run on multiple platforms including Android, iOS, and Windows Phone. Key differences between Cordova 2.x and 3.x include plugins no longer being included and changes to platforms. Cordova uses a command line interface to automate project setup and requires configuration files and packages.json. The config.xml file follows W3C widget standards and is important for native platform configuration. Developers can use IDEs like Eclipse or Xcode for native tools or cloud-based build services like PhoneGap Build.