PDE stands for Plug-in development environment. The PDE project provides a number of views and editors that make is easier to build plug-ins for Eclipse. Using the PDE, you can create your plug-in manifest file (plugin.xml), specify your plug-in runtime and other required plug-ins, define extension points, including their specific markup, associate XML Schema files with the extension point markup so extensions can be validated, create extensions on other plug-in extension points, etc. The PDE makes integrating plug-ins easy and fun. Eclipse PDE home is here.