MODE %|<your nick>|<channel> [<mode> [<mode parameters>]]

Common channel modes are:

     i - Invite only - People can't join to channel without being
         /INVITEd, or being in invite list (+I, see below).
     m - Moderated - People who don't have voices (+v) can't send
         messages to channel
     p - Private - People who aren't joined to channel can't see it
         for example with /WHOISing people who are in channel.
     s - Secret - Like private, but the channel isn't displayed in
         /LIST's output.
     n - No external msgs - Without this mode, anyone can send messages
         to channel without even being joined.
     t - Topic can be changed only by channel operators.

     k <key> - Channel password (aka. key) - The channel can't be joined
               without specifying the channel key (see section 6.2).

     l <count> - User limit - No more than <count> people can join to
                 channel. This can be overridden with /INVITE with some
                 servers.

                 This is usually used for protecting channel from join
                 flooding, like some bot allows max. 5 users to join in
                 one minute or so.

     b - Set/remove ban. For example MODE #channel +b *!*@*.org bans
         everyone from .org domain.

         If someone from .org domain was already in channel before the
         ban was set, he/she cannot send any messages to channel (doesn't
         work with all servers).

         Bans can also be overridden with /INVITE, although many stupid
         IRC clients automatically kick the user out because they see
         the ban and think that because of it the user shouldn't be in
         the channel (doesn't work with all servers).

     e - Ban exceptions. You could for example ban everyone from
         *!*@*.org but set ban exception to *!*@*.host.org - does not work
         with all servers.

     I - Invite list. If channel is invite only (+i), people in this
         list can join it without being /INVITEd - does not work with all
         servers.

         This is excellent for in-country channels that don't want
         foreigners (spammers!) to join the channel, for example setting
         channel's mode to +i and +I *!*@*.fi allows only finnish people
         to join the channel. In addition to this, there's usually a bot
         in the channels and sending /MSG bot invite command to it
         /INVITEs you to the channel.

         On IRCnet, the ':' feature in channel names can also be used for
         a similar effect, see section 6.2.

     o <nick> - Grant or revoke channel operator status from nick
     v <nick> - Grant or revoke voice status from nick, only people with
                +v (or +o) can talk to channel when it's moderated (+m).

You can send multiple mode changes with one mode command:

/MODE #channel +nto-o+v nick1 nick2 nick3

This would set channel's mode to +nt, give ops to nick1, take ops from nick2 and give voices to nick3.

You can set only a limited number of modes that require an argument in one command. In IRCnet it's 3, in EFnet it's 4 and in many others it's 6. If it's not known, Irssi defaults to 3. Irssi will also automatically split them, so you can use /MODE +oooooo n1 n2 ... command to op 6 people and Irssi will split it to two commands in IRCnet/EFnet.

Many networks have additional modes and/or change the meaning of existing modes. Check the documentation for the network or the server software in use for details.

See also: OP, DEOP, VOICE, DEVOICE, BAN, UNBAN