1) BNF [0] is a notation [1] for formally defining languages [2], e.g. the syntax [3] of programming languages like C [4] or Java [5], or in this case, the IRC protocol. Every BNF definition is a bunch of strings. BNF is not a data model [6][7], a serialization format or a language for representing IRC messages (as XML is for XMPP). You are correct in that IRC is a much simpler protocol than IRC, though.

2) There is no difference between commands and messages in IRC, because to send a message, the command PRIVMSG [8] is invoked:

  JOIN #channel
  NAMES #channel
  PRIVMSG #channel :Hello interwebs!

The fact that commands in IRC clients begin with a slash is a UI convention, and unrelated to the protocol.

[0] http://en.wikipedia.org/wiki/Backus%E2%80%93Naur_Form

[1] http://en.wikipedia.org/wiki/Mathematical_notation

[2] http://en.wikipedia.org/wiki/Formal_language

[3] http://en.wikipedia.org/wiki/Syntax_(programming_languages)

[4] http://www.cs.wmich.edu/~gupta/teaching/cs4850/sumII06/The%2...

[5] http://www.cs.au.dk/~stm/RegAut/JavaBNF.html

[6] http://en.wikipedia.org/wiki/Abstract_data_type

[7] http://en.wikipedia.org/wiki/Data_structure

[8] http://tools.ietf.org/html/rfc1459.html#section-4.4