Wikipedia
GConf was a system used by the GNOME desktop environment for storing configuration settings for the desktop and applications. It is similar to the Windows Registry.
It was deprecated as part of the GNOME 3 transition. Migration to its replacement, GSettings and dconf, is ongoing.
Changes to this system are controlled by GConfd, a daemon. GConfd watches out for changes to the database, and when they are changed, it applies the new settings to applications using it. This technology is known as "auto-apply", compared to "explicit-apply", which requires users to press an OK or Apply button to make changes come into effect. The term "instant-apply" is sometimes used, compared to plain "apply".
The GConf database by default uses a system of directories and XML files, stored in a directory called ~/.gconf. GConf can also use other backends, such as a database server, but XML file storage is the most common configuration.
The application gconf-editor is provided to allow users to change settings manually, but it is not usually used for end-user preferences. These should be provided by the application, but hasn't in some cases, causing upset among users (see main gconf-editor article for details).