System tray icons generally fall into four categories:
Applications registering with the system tray provide one of those categories. An Instant Messaging application or an Email client will fall into 'Communications'. System services are for example software update notifications. Examples for Hardware are a battery monitor a mixer or a Network Management applet. Applications that use the system tray for freeing space in the taskbar use the Application Status category.
A System Tray Icon will Have the following elements and properties.
The interactions can be considered in terms of the 'Client' ie the icon itself, and the 'Host' eg. the system tray.
A client of the system tray is an application that registers with the system tray to display its information. The system tray (the host) reacts to signals it receives from the client. The host also can pass back an action to the client application, for example when the application has been activated. The host receives the signals in an asynchronous fashion from the client.