.rncis the suffix generally chosen for files which are instances of RNC. "RELAX" is glossed as, "REgular LAnguage for Xml". The "NG" part is the expected, "Next Generation". The Wikipedia page is appropriate, if brief.
Yutaka Furubayahi's Relax NG page includes links to the RNC Tutorial, specifications, the RNG book, comparisons of RNG to XSD, Jing and other validators, Trang and other transformers, and much else.
"Converting DTDs (and DTD developers) to RELAX NG Schemas"
I spend a lot of time with the XML schema part on datatypes.
"The Importance of Being Valid" is a breezy introduction to RNC and XSD. I also wrote "Which of These 5 XML Tactics is Missing From Your E-Commerce?"
Valuable comments from James Clark in fall 2009.