ASP.NET introduced new declarative expression syntax which allows you to substitute values into page at run time called Expression builders.
There are five type of expression builder in asp.net
AppSettingsExpressionBuilder
ConnectionStringsExpressionBuilder
ResourceExpressionBuilder
AppSettingsExpressionBuilder
ConnectionStringsExpressionBuilder
I have found a great article that describes the this expression builder in details.
here is the link..
http://www.beansoftware.com/ASP.NET-Tutorials/Expression-Builder.aspx