Report Definition Language (RDL) is an XML-based schema for defining reports. The goal of RDL is to promote the interoperability of commercial reporting products by defining a common schema that allows interchange of report definitions. To encourage interoperability, RDL includes the notion of compliance levels that products may choose to support.
Download the RDL Specifications for SQL Server by clicking the links below.
Abstract courtesy : Microsoft
Reference: Pinal Dave (http://blog.SQLAuthority.com)