/topic When you've started a new channel, use this command to let others know what it's about.
/topic #Amiga
would tell people who use /list that your channel is meant for discussing Amiga computers.
/who <chan> Shows you the e-mail address of people on a particular channel.
/who #foo
would show you the addresses of everybody on channel foo.
/who
by itself shows you every e-mail address for every person on IRC at the time, although be careful: on a busy night you might get a list of 500 names!
/whois Use this to get some information about a specific IRC user or to see who is online.
/whois nickname