Если для чайников, то действительно различий нет...
А так:
Q: What is the difference between RDL and RDLC formats?

A: RDL files are created by the SQL Server 2005 version of Report Designer. RDLC files are created by the Visual Studio 2005 version of Report Designer.

RDL and RDLC formats have the same XML schema. However, in RDLC files, some values (such as query text) are allowed to be empty, which means that they are not immediately ready to be published to a Report Server. The missing values can be entered by opening the RDLC file using the SQL Server 2005 version of Report Designer. (You have to rename .rdlc to .rdl first.)

RDL files are fully compatible with the ReportViewer control runtime. However, RDL files do not contain some information that the design-time of the ReportViewer control depends on for automatically generating data-binding code. By manually binding data, RDL files can be used in the ReportViewer control.

Note that the ReportViewer control does not contain any logic for connecting to databases or executing queries. By separating out such logic, the ReportViewer has been made compatible with all data sources, including non-database data sources. However this means that when an RDL file is used by the ReportViewer control, the SQL related information in the RDL file is simply ignored by the control. It is the host application's responsibility to connect to databases, execute queries and supply data to the ReportViewer control in the form of ADO.NET DataTables.

Если умеете создавать RDL или знаете, где про них почитать, то так и делайте. Потом измените расширение (про возможность таких действий есть в MSDN). По дизайну ну совершенно различий нет. На вскидку, по-моему можно создать Report Project в MS Visual Studio 2005, там получите RDL. Чтоб сразу RDLC - ну в WindowsApplication бросьте ReportViewer на форму, там в правом верхнем углу найдите маленькую стрелочку, нажмете, получите выпадающий список и выберете эквивалент 'Создать новый отчет' на английском языке - дословно не помню, автоматом получится RDLC. Ну вот еще сайт www.gotreportviewer.com посмотрите, там про компоненту Reportviewer на начальном уровне и про дизайн. Остальное к MSDN и гуглу.