Notification Services is a programming framework based on XML and T-SQL. It is used to create applications that generate and send messages to subscribers.
Notifications can be sent according to a schedule, or in response to conditions or events. Notifications can be sent to a wide variety of devices. For example, a notification can be sent to a user's cellular phone, personal digital assistant (PDA), Microsoft Windows Messenger, or e-mail account. Because these devices often accompany the user, notifications are ideal for sending high-priority information.
We can create the notification Services in 2 ways:
The first would be to build everything from scratch, second is to use the SQL Server 2005 Notification Services (SQLNS), which is a new API, intended to help you build notification applications. SQLNS applications are always hosted inside the SQL Server or inside a Windows Service.