Wikipedia
freedesktop.org (fd.o) is a project to work on interoperability and shared base technology for free software desktop environments for the X Window System (X11) on Linux and other Unix-like operating systems. It was founded by Havoc Pennington from Red Hat in March 2000. The project is hosted by Software in the Public Interest, the non-profit organization created by the Debian Project.
There are many development frameworks for X, and this is unlikely to change. The organisation seeks to ensure that differences in development frameworks are not user-visible.
Widely used open-source X desktop projects—such as GNOME, KDE, and Xfce—are collaborating with the freedesktop.org project. In 2006, the project released Portland 1.0 (xdg-utils), a set of common interfaces for desktop environments.
freedesktop.org was formerly known as the X Desktop Group, and the abbreviation "XDG" remains common in their work.