According to the Eclipse help,

1)Project Explorer: The Project Explorer provides a hierarchical view of the artifacts in the Workbench, which is customized by the specific configuration of your Workbench.

2)Package Explorer - Provided by the Java Development Tools (JDT) UI project, this provides a view of Java classes for the workspace. Generally speaking, the presentation of the Package Explorer and Project Explorer for Java objects is substantially similar. The JDT is not used for projects beyond Java.

3)Navigator - This view is an implementation of the now deprecated (as of 3.5) org.eclipse.ui.views.navigator.ResourceNavigator class provided in the org.eclipse.ui.ide plugin. It shows only the workspace resources and does not support extensibility to show other content. In a future release, the Navigator view will be provided by an implementation of the CommonNavigator.