Xfce is the best software desktop environment for GNU/Linux systems ever. It is sexy, lightweight, fast, stable and simple yet it has more features and configuration options than GNOME. It is heavier than Fluxbox yet it performs very well on older hardware.
Xfce is based on the GTK+ v2 toolkit. It consists of separate components who work together to form a very easy to (ab)use Windows 95-like desktop.
The desktop has panels and these panels can be placed anywhere. Numerous widgets and buttons can be put on these panels. They can and should be auto-hidden, and the auto-hide feature actually works (unlike KDE).
Numerous simple desktop applications meant to be used on the Xfce desktop are made specifically for it. These applications can be used on any desktop environment, they are not tied to Xfce.
The Xfce applications are mostly very simple programs and some of them have a disturbing lack of simple features which makes them useless (see gpicview).