Query Notifications is a powerful new feature, built in to SQL 2005, usable from ADO.NET 2.0 and from ASP.Net 2.0 directly.
Query Notifications allows you send a query to SQL server and request that a notification be generated if any row included in the query is changed.
Commands sent to server may include a tag that requires a notification. Whe the server sees this tag, it will create a notification subscription that fires once the result set is change for the query statement.
This is extremely useful for client side caching of results from database server. With this feature, you don’t have to pulling data periodically.