Taken from http://www.irchelp.org/irchelp/rfc/
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RFC: Internet Relay Chat Protocol

RFC stands for Request for Comments. As defined by FOLDOC, an RFC is an 
Internet information document or standard. The IRC RFCs provide the technical 
details that describe the Internet Relay Chat protocol. They are must-reads for 
anybody interested in writing an IRC client or advanced script.

RFC 1459: Internet Relay Chat Protocol
    This is the original and authoritative RFC for IRC. Written in 1993, it is 
    the basis for IRC as we know it. If you want to write an IRC client, you 
    need to read this carefully. It does not include details on DCC or CTCP, 
    which came later and are described in the following. 

The Client-To-Client Protocol (CTCP)
    Written in 1994, the CTCP specification follows the original IRC RFC above, 
    and it is the authoritative document for the client-to-client protocol. 
    It includes the related, older DCC specification in its entirety. Later, 
    an unofficial Internet Draft was released for an updated CTCP specification 
    and has been adopted by many major clients including mIRC. 

New RFCs for IRC
    In 2000, these 4 new RFCs were created to address many of the changes that 
    took place since the original was written. You should still consider these 
    as updates rather than replacements for the original RFC 1459. In 
    particular, they do not necessarily represent a new, official standard.

        * RFC 2810 Internet Relay Chat: Architecture
        * RFC 2811 Internet Relay Chat: Channel Management
        * RFC 2812 Internet Relay Chat: Client Protocol
        * RFC 2813 Internet Relay Chat: Server Protocol