WebSphere Integration Developer (WID) is IBM’s development tool for constructing SCA modules that can be deployed to WebSphere ESB and WebSphere Process Server. It is built on top of Rational Application Developer (RAD), which is in turn Eclipse-based, so it inherits a lot of functionality, including support for Java, J2EE, UML, etc. However, it also provides the ability to develop:
ESB SCA-based mediation modules, which can contain ESB mediation flows (and Java components).
Process Server SCA-based modules, which can contain Process Server SCA components such as business processes (using BPEL), human tasks, business state machines, business rules, and selectors.
(see this posting for more information on SCA modules and components)
WID, like RAD, is Eclipse-based, and provides the same flexible, customizable development environment. For example, one can customize the Eclipse Capabilities provided by WID so that the Process Server and/or ESB development functionality is hidden (once loaded, go to Window / Preferences / Workbench / Capabilities / Integration Developer / *). This is useful if you know you want to target a particular type of server.
Although WID is often used with WebSphere ESB or Process Server, it is not supplied in the box with them and must be obtained separately. However, it does contain a version of Process Server that can be used for testing modules before deployment - commonly referred to as the Integrated Test Environment (ITE) or Unit Test Environment (UTE). During installation of WID, you can create an ESB or Process Server profile against this for testing with (or both). This is possible because Process Server contains a superset of the functionality of ESB. See this Developerworks article for more information on testing this way, as well as some more information on WID.