Every desktop environment on Linux has its own notification system which implements the Freedesktop notifications specifications. Some of them, like GNOME or KDE, use their own built-in notification systems which cannot be replaced; others like Xfce or Mate, use more modular components (Xfce notification daemon and Mate notification daemon, respectively). Desktop-independent notification systems also exist (dunst, for example): most of the time they are used on minimal setups (e.g. when using a plain window manager instead of full blown Desktop environments).
