The xdg_shell protocol defines new xdg_surface and xdg_popup interfaces. The xdg_surface provides a desktop-style window with move, resize, and maximization support. The xdg_popup interface provides a desktop-style pop-up/menu.
The premise of xdg_shell is more sharing of Wayland-related code and concepts amongst desktop environment developers with those implementing their own compositors for their desktop user-interfaces. Xdg-shell in relation to Wayland has been compared in nature to EWMH (Extended Window Manager Hints) to X11.
Xdg-shell is used in place of wl_shell, the core Wayland protocol's shell interface. The xdg-shell protocol isn't proposed for merging into the core Wayland protocol and is not yet a stable protocol. The xdg-shell protocol will grow as more desktops are ported to Wayland and new needs of it are discovered. The xdg-shell protocol already has input and support from GNOME and KDE desktop developers.
The set of 16 patches sent in this morning by Intel's Rafael Antognolli provide the XML-based xdg-shell protocol, an implementation of xdg-shell for the Weston compositor, and a client implementation for Wayland's toytoolkit. These new xdg-shell patches can be viewed on Wayland-devel.