Some programming languages and tools allow cross-platform app support (i.e., for both Android and iOS). Third-party tools, development environments, and language support have also continued to evolve and expand since the initial SDK was released in 2008. The official Android app distribution mechanism for end users is Google Play; it also allows staged, gradual app releases as well as the distribution of pre-release app versions to testers.
Android Studio is the official, unified, integrated development environment (IDE) for building apps designed to run on Android devices.
ADT (Android Developer Tools) is a plugin for Eclipse that provides a suite of tools that are integrated with the Eclipse IDE. It offers you access to many features that help you develop Android applications quickly. ADT provides GUI access to many of the command line SDK tools as well as a UI design tool for rapid prototyping, designing, and building of your application’s user interface.
Because ADT is a plugin for Eclipse, you get the functionality of a well-established IDE, along with Android-specific features that are bundled with ADT.