At first,If you want to develope a android application,you must have its SDK,short for Software Development Kit, 即软件开发工具包.It is the same with Java .JDK.
SDK Manager:You know that there are many versions of android platform ,from 1.5 to 4.1 now.So for different systems ,we may support on different versions.SDK Manager is a tool that make you view which version and tools you have installed.
After you have installed SDK platform android 2.2 by SDK Manager,you can go to the path Android SDK instaled,for me(D:\Android\android-sdk),and in the folder platform you can find a new subfolder named android-8(the API version of andord 2.2).
ADT:Android Development Tools (ADT) is a plugin for the Eclipse IDE that is designed to give you a powerful, integrated environment in which to build Android applications.
ADT extends the capabilities of Eclipse to let you quickly set up new Android projects, create an application UI, add packages based on the Android Framework API, debug your applications using the Android SDK tools, and even export signed (or unsigned) .apk files in order to distribute your application.
SDK platform android 2.2:You may see this after run SDK Manager,Yes,It's the version of Android Virtural Device.You can select different versions and install it for your application.