Request For Comments - the extremely modest name for a series of standards (defined under the auspices of the IETF) that define most of the important things (e.g. the TCP, IP, SMTP, FTP, and HTTP protocols) about how the Internet works.
Official home: http://www.rfc-editor.org/
[ List some RFCs which is likely to be of interest, such as those defining HTTP, mime, etc. ]