What is ADT (Android Development Tools)

The ADT (or Android Development Tools) is an extension to the Eclipse IDE (Integrated Development Environment).

The ADT plugin has support for the creation and debugging of Android applications. 

Here are some of the features you could use in Eclipse after installing the ADT:

• create new and manage existing Android application projects
• compile and debug Android applications
• access the tools for accessing your Android emulators and devices
• export Android applications into Android Packages (APKs)
• create digital certificates and sign APK files

How to install Android Development Tools (ADT)

1) Run Eclipse

2) Select Help > Install New Software.

3) Enter https://dl-ssl.google.com/android/eclipse/ and hit Enter.

4) You'll see the Developer Tools item and you need to expand it to see its content (e.g. Android Development Tools, Android DDMS, Android Hierarchy View, Android Traceview). 

5) Select all items and click Next button twice.

Note: If you have any problems downloading the ADT, consult Google's help at http://developer.android.com/sdk/eclipse-adt.html#installing.

6) Accept the offered licenses - select "I accept the terms of the license agreements" and click Finish button.

7) Restart Eclipse after the installation is finished.

8) When Eclipse is restarted, you will be asked to configure your Android SDK. Because the Android SDK has already been downloaded earlier in the previous section, check the "Use existing SDKs" option and specify the directory where you have installed the Android SDK. 

9) Click Next. After this step, you will be asked to send your usage statistics to Google. Once you have selected your choice, click Finish.

Note: The installation steps could be slightly different in different SDK versions, so if you don't experience the same steps as we described, just follow the instructios on screen.

Full Android development environment setup tutorial read here.