GWT: Difference between gwt-user.jar and gwt-dev.jar?

What is the difference between gwt-user.jar and gwt-dev.jar? Looking at their contents, I cannot see why they are packaged as they are.

I am wondering especially how to use the two JARs for compilation and deployment.
Which of the two do I need ...

1. ... for running javac (.java to .class)?
2. ... for running the GWT compiler (.class to JavaScript)?
3. ... for running GWT development mode and debugging?
4. ... for deploying my app to Google App Engine?
5. ... to have in my Eclipse project's classpath?

Any clarification on these uses or, even better, an explanation of why that is, would be greatly appreciated.