@SYNTAX:mode@

Changes and/or views modes. You can either change
your own modes or channel modes (given that you're
a channel operator).

Examples:

/MODE * +s    
   - Makes the current channel secret

/MODE #irssi -o Beel0  
   - Deops user Beel0 on channel #irssi

/MODE * +b *!*@*.org
   - Bans on the current channel all users coming 
     from .org hosts.

/MODE yournick +w
   - Makes you to see the wallops.

See also: BAN, VOICE, OP, DEOP