The first thing to understand in the registry is the terminology. An extension is a contributed functionality that is often found in the plugin.xml file as an <extension> element. The extension itself provides some configuration or customization that can be processed appropriately. An extension is like a USB device, such as a mouse or keyboard. For example, the new feed wizard was added as an extension in the previous chapter:

<extension point="org.eclipse.ui.newWizards">
  <category name="Feeds" id="com.packtpub.e4.advanced.feeds.ui.category"/>
</extension>

An extension point defines the contract of an extension, along with any required arguments or attributes that an extension must provide. An extension point is like a USB hub that allows extensions (USB devices) to be plugged in. For example, the newWizards extension point is defined in the plugin.xml file of the org.eclipse.ui plug-in as follows:

<extension-point id="newWizards...