The freedesktop.org Desktop Notifications Specification provides a standard way for applications to display pop-up notifications. These are designed to make you aware of something, without interrupting your work with a window you must close.

Notify OSD presents these notifications as ephemeral overlays, which can be clicked through so they don't block your work. It queues notifications, to prevent them from flooding your screen. And as well as handling standard notification updates, Notify OSD introduces the idea of appending — allowing notifications to grow over time, for example in the case of instant messages from a particular person.

Notify OSD design specification: <https://wiki.ubuntu.com/NotifyOSD>
Desktop Notifications Specification: <http://www.galago-project.org/specs/notification/0.9/>
Development guidelines for application developers: <https://wiki.ubuntu.com/NotificationDevelopmentGuidelines>

Notify OSD is one of several mechanisms developers can use to draw attention or seek a decision.
<https://wiki.ubuntu.com/NotificationDesignGuidelines>