The major GWT components include:

GWT Java-to-JavaScript Compiler

Translates the Java programming language to the JavaScript programming language.

GWT Development Mode

Allows the developers to run and execute GWT applications in development mode (the app runs as Java in the JVM without compiling to JavaScript). Prior to 2.0, GWT hosted mode provided a special-purpose "hosted browser" to debug your GWT code. In 2.0, the web page being debugged is viewed within a regular browser. Development mode is supported through the use of a native-code plugin called the Google Web Toolkit Developer Plugin for many popular browsers.

JRE emulation library

JavaScript implementations of the commonly used classes in the Java standard class library (such as most of the java.lang package classes and a subset of the java.util package classes).

GWT Web UI class library

A set of custom interfaces and classes for creating widgets.