The Android SDK provides you the API libraries and developer tools necessary to build, test, and debug apps for Android.
It is recommend to download the ADT Bundle to quickly start developing apps. It includes the essential Android SDK components and a version of the Eclipse IDE with built-in ADT (Android Developer Tools) to streamline your Android app development.
Download ADT Bundle http://developer.android.com/sdk/index.html
Android offers a custom plugin for the Eclipse IDE, called Android Development Tools (ADT). This plugin provides a powerful, integrated environment in which to develop Android apps. It extends the capabilities of Eclipse to let you quickly set up new Android projects, build an app UI, debug your app, and export signed (or unsigned) app packages (APKs) for distribution.
If you need to install Eclipse, you can download it from eclipse.org/downloads/.
Note
If you prefer to work in a different IDE, you do not need to install Eclipse or ADT. Instead, you can directly use the SDK tools to build and debug your application.
Your Eclipse IDE is now set up to develop Android apps, but you need to add the latest SDK platform tools and an Android platform to your environment.
References : http://developer.android.com/sdk/index.html