Ntfy is a free and open source notification service written in Go which lets us easily send and receive push notifications on smartphones or desktop computers via simple POST or PUT requests. The basic online service is publicly available free of charge and, on Linux, it is also possible to self-host an Ntfy instance.
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).