From OpenNMS

It's all very well for OpenNMS to know that something significant has happened, but ultimately humans need to be told. A given notification is triggered to be sent when a specific event occurs. Filtering can based on any event parameter, such as the node on which it occurs. For example, you might configure different notifications for two sets of servers, to notify an appropriate set of users depending on which server is affected.

Further, notifications have escalations; if a notification is not acknowledged (through the web interface) within a configured period of time, it is sent to the next destination on the escalation, and so on up the list. Notifications can be sent by e-mail, SMS text message, page, or by any other method for which a script can be written.