Debian has two menu systems:
The so called Debian menu system provides a centralized list of all installed user applications (for which the packager provided a menu entry). It is a documented in the DebianPolicy. Those menu entries are available in most window manager / Desktop environment in Debian (KDE, GNOME, ?GNUStep, ?Enlightenment and ?Windowmaker, making every application available in every environment). Each application drop a menu entry in /usr/share/menu/ and menu manager register to menu by droping an entry in /etc/menu-methods/.
The Freedesktop Desktop Menu Specification, which was adopted as the primary menu in Gnome, KDE, LXDE and others. It is famous for the *.desktop files in /usr/share/applications