Our Command reference includes information on the commands you can use on IRC servers.
Our Numeric reference includes information on the format of numerics sent by servers, and how to interpret them.
Our Glossary includes definitions for terms used in the IRC development and user community.
The IRCv3 Working Group aims to improve and extend the IRC Client Protocol, and their specification documents are good reading for any IRC developer.
The ircdocs references include additional information on many IRC subjects, as well as linking to other community documents and reference material.