An Acronym for Request For Comments. A set of specifications published by the IETF.

Internet Standards, Information, Best Common Practises, and other related stuff. Heavy reading, but if you want to know how the Internet works, there is no other way.

Also has some humorous ones, like RFC:1149 or HTCPCP/RFC:2324.

Of particular interest is the Best Current Practice sub series: BCP

Wiki handles RFCs, to refer to an RFC like so, RFC:1918, just type !RFC:1918

A good place to look at RFCs is as it tells you whether the one you are looking at has been replaced by a later one.