The Network Message Action is equivalent to doing a "net send" from the command line. It allows you to direct a message box pop-up to any particular user or computer on the network.
The client machine must be running Microsoft's Messenger service to receive and display these messages. Because of spam and security concerns, the Messenger service is not started by default on most systems.
Pressing the Message button displays the configuration dialog below. This lets you customize message text, select different templates to use, and to create new templates. This is most useful when sending alerts to devices like pagers and cell phones which might only accept the first sentence or two of a message. Also, having the abillity to use different message templates will help you get the right information to the right groups.
You can also specify specify that the message is HTML, and enter an HTML message template. Enclose the template in an <html> tag. Don't bother with a <head> tag as most email clients will strip it out.
Some good hints and tips about HTML email are available here: http://www.mailchimp.com/resources/guides/email-marketing-field-guide/
You can also rename the action as it shows up in the various action lists (for example to give the email action a group name). You can reset the action to its original/default name by simply clearing the name field.
A typical alert email could look something like this:
Note: Actual message content will vary depending on the product being used, and the monitor which fires the actions.