I thought this was a really interesting and detailed article:

Microsoft Office SharePoint Server (MOSS) 2007 ships with protectors based on the Information Rights Management (IRM) framework. These provide Microsoft Office users with the benefit of SharePoint integration with Active Directory Rights Management Services (AD RMS). Unfortunately, Windows SharePoint Services ( WSS ) 3.0 does not include IRM protectors out of the box. However, there is a solution. If you browse the sample applications and code snippets on the MSDN Code Gallery, among the gems you'll find is the Microsoft Office File Format Protectors source code, which Microsoft published in August 2008. The source code comes with a non-exclusive, worldwide, royalty-free "as-is" Microsoft Public License (Ms-PL). This is a great find because it means you can compile the source code and add these components to WSS 3.0 as well. In this way, you can benefit from AD RMS-based security and compliance features in any SharePoint environment.