RDL - RDL stands for Report Definition Language
RDLC - Report Definition Language Client-side processing
RDL files are created by the SQL Server 2008 version of Report Designer. RDLC files are created by the Visual Studio 2008 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 2008 version of Report Designer. (You have to rename .rdlc to .rdl first.)