Notification Services applications are based on a subscriber/subscription model.
The Key components in a Notification Services application are
A user or application that requests and receives the notification
A request for information, delivery mechanism, and destination for requested notifications. For example change of a stock price.
A piece of information, in which the subscriber is interested. For example as soon as a stock price has changed an event can be generated that's processed within SQLNS.
A message containing the information requested by the subscriber in the subscription