notify-send is a command to send desktop notifications to a notification daemon. It is included in libnotify-bin package, which is a library that sends desktop notifications to a notification daemon, as defined in the Desktop Notifications spec. These notifications can be used to inform the user about an event or display some form of information without getting in the user's way.
You can install it using Synaptic Package Manager by searching for libnotify-bin package and install it. Or you can install it via terminal using this command:
apogee@apogee-ubuntu:~$ sudo apt-get install libnotify-bin
And then, you can use the notify-send command as described in my previous post about Custom Gnome Notification for your apps. Have fun!!