Plugin = A collection of code that follows a predefined API and has an XML file or two that describes what it does, where it hooks in and stuff like that. The platform reads this XML file, understands the plugin and hooks it into the system.
Eclipse = A platform to build applications on. The platform is crafted in such a way that there is core set of APIs that provide basic functionalities and super-flexile/powerful plugin system. The idea with Eclipse is that almost ALL the functionality (text editor, preferences, searching, running JUnit, etc.) is a plugin that is hooked into the system via those XML descriptors I mentioned. You can build an Email client using Eclipse. People have built all sorts of applications with Eclipse, it’s a flexible platform (and huge).
Eclipse JDT = A big collection of plugins written ontop of Eclipse that provide Java-specific functionalities. Because people *mostly* use Eclipse as a Java IDE, they tend to call it “Eclipse”, like I just did. But they really mean “Eclipse JDT”, since that is the collection of plugins that provide the Java functionality.
MyEclipse = A big collection of J2EE-specific plugins that provide all the web-specific developing capabilities.
So what you need to compare is:
JDeveloper 10G against (Eclipse SDK + MyEclipse)