Here are the main parts of the official GIMP website.
www.gimp.org is the official GIMP site. If you know only one site, it must be this one, especially because it contains links to many other important sites. On the front page, you’ll find news about the latest releases and links to pages where you can download them. The rest of the site contains the major pages mentioned in this section.
wiki.gimp.org is the GIMP Developer wiki, a collaborative website about GIMP. It’s intended for developers but helpful to anyone who wants to know about developers’ projects.
bugs.gimp.org is a somewhat esoteric page. It does not explain how to report bugs, but only enumerates the current list of bugs in various versions of GIMP. This page lets you look at the work of the developers and get to know who is working on what. To file a bug, take a look at www.gimp.org/bugs/.
developer.gimp.org is about GIMP development. It is not really updated presently, and its main interesting feature is the tutorial about writing a plug-in at developer.gimp.org/writing-a-plug-in/1/.
gui.gimp.org is a working wiki about the GIMP graphical user interface (GUI). This site explains how the work for this GUI is done and in what direction it is progressing.
docs.gimp.org, surprisingly, is not the same as www.gimp.org/docs/. In fact, the first link leads to a page that contains links to the manuals of preceding versions, together with the present one, which is currently a work in progress. The second link leads to the current (finished) documentation and can be accessed directly from GIMP itself. It will point to the documentation of version 2.6 until the documentation of version 2.8 is finished. The documentation is currently available in English, Dutch, French, German, Italian, Norwegian, Korean, Russian, Spanish, and Swedish. For 2.8 it will also include Greek and Japanese.
registry.gimp.org is the GIMP Plugin Registry, where all plug-ins and scripts built by GIMP users are filed and available for download. The GIMP Plugin Registry takes the form of a blog, with the most recent entries appearing on the front page. You’ll also find forums, where you can read and leave comments, and most useful of all, a search engine for hunting for a precise plug-in. This website is probably the second most useful site, after www.gimp.org.