Eclipse is a plugin based platform and this is well explained in its Platform architecture user guide section. As application developers, we normally use one of the pre-packaged versions (for example Eclipse IDE for Java Developers) and don’t need to use its SDK. But if you want/need to extend Eclipse, you can easily install PDE (Plug-in Development Environment)on top of your current installed package instead of downloading another package.
To install Eclipse Plug-in Development Environment (PDE), you simply open your Eclipse and go to Help->Install New Software…->Drop down “Work with” and select “Kepler – http://download.eclipse.org/releases/kepler ” (or you can enter the url if not present)->Enter “Eclipse plug-in development environment” as the filter->select and install it.
After the installation, you can follow a very good tutorial by Lars Vogel to start your plugin development. And you can always refer Eclipse official Platform Plug-in Developer Guide for more information.