A standardised daemon and API for storing configuration information.
Information stored under a path into a registry-like structure.
Originally designed to store parameters for running applications, not so much for persistent state.
That is changing a bit now, since there is nothing else to server the "persistent state" requirement.
Best description and overview of GConf is Havoc Pennington's paper from OLS 2002.
Do not be tempted to use gnome-config in place of GConf. The former is not safe except when the user is only logged in once and running a single copy of the application.