GWT Development Notes

General

The module XML file contains definition of the GWT module, the collection of resources that comprise a GWT application or a shared package. The default XML inherits the core GWT functionality required for every project. Optionally, you can specify other GWT modules to inherit from.

In the module XML file, you specify your application's entry point class. In order to compile, a GWT module must specify an entry point. If a GWT module has no entry point, then it can only be inherited by other modules. It is possible to include other modules that have entry points specified in their module XML files. If so, then your module would have multiple entry points. Each entry point is executed in sequence.

UI Components

Whenever possible, GWT defers to browsers' native user interface elements.
This means that GWT buttons render appropriately in different browsers and on different client operating systems. The benefit of using native browser controls is that they are fast, accessible, and most familiar to users. Also, they can be styled with CSS.  

Root Panel:
A Root panel is the container for the dynamics elements of your application. It is at the top of any GWT user interface hierarchy. There are two ways you can use a Root panel, either to generate the entire body of the page or to generate specific elements embedded in the body. The Root panel works by wrapping an element in the HTML host page. By default (that is, if you don't add any placeholders in the host page) the Root panel wraps the body element. However, you can wrap any element if you name it and then, when you call the Root panel, pass the name as a parameter.

Debugging
One benefit of using GWT in your AJAX application development is that you can see the effects of your code changes as soon as you refresh the browser running development mode 

http://code.google.com/webtoolkit/doc/1.6/tutorial/buildui.html (part 3)