Apparently John Chambers added Reference Classes to R in version 2.12. There doesn\'t appear to be much information online yet, but they\'re calling them R5 classes, which
The request for documentation for ReferenceClasses comes up every now and then, for example on the r-devel list. The best answer so far is to actually look at what help(ReferenceClasses)
gives you---which is a pretty decent start.
Then there are a few presentations:
And as noted by mdsummer in the comment, R5 was a short-lived joke. There was already another R-related project called R5, and John much prefers ReferenceClasses. And I reckon they are here to stay. People use them already as e.g. Jeff Horner is his new Rack package.