This ExtReportViewer webpart contains Microsoft.Reporting.WebForms.ReportViewer inside and is intended for SharePoint Foundation with Reporting Services run in Native mode.
The main advantage of ExtReportViewer over standard Microsoft ReportViewer plug-ins for both Native and Integrated SSRS is its sophisticated connection interfaces.
ExtReportViewer can be both consumer and provider for other linked reports of the same type and allows creating reports chain where one main report supplies valid parameters for a number of other secondary reports.
ExtReportViewer can also be a consumer for multiple HTML Form webparts. It can even receive parameters from both SPList and, after that to be updated from HTML Form. HTML Form parameters always override List parameters.