Hypersmurf
Moderatarrrrh...
Check the PHB glossary.
And don't forget to add his "shield bonus" while you're at it.
The glossary is a quick reference - it doesn't always give complete information. It's often necessary to check the relevant section for full details.
Example: Take 10 and Take 20. They're defined in the glossary, but to fully understand how they work, you need to look them up in the main section of the PHB.
In similar fashion, the glossary gives a quick overview of DR, but if you want to understand how it works, you read the DMG.
-Hyp.