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.netAppSettingsExpressionBuilderConnectionStringsExpressionBuilderResourceExpressionBuilderAppSettingsExpressionBuilderConnectionStringsExpressionBuilderI 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