Official Plugins are plugins that are included in the official Geeklog distribution and considered part of Geeklog.
Historically, the plugins that came bundled with Geeklog were the Calendar, Links, Polls, Spam-X, and Static Pages plugins. Although technically plugins, they were pretty much tied to Geeklog's install and upgrade process and couldn't really be used with any Geeklog version other than the one they were shipping with.
As of Geeklog 1.6.0, the coupling of Geeklog and the bundled plugins has been loosened enough so that it is now possible to remove the plugins from the tarball without the install or upgrade failing. It is also possible now to add other plugins and have them installed automatically when Geeklog is installed.
Geeklog 1.6.0 also introduced a new "official" plugin, XMLSitemap Plugin.