Channel Modes

A channel mode is a 'flag' that can be set on a channel.

Flag Information
+b [pattern] Set a ban, based upon a pattern. The patterns follow standard 'glob' patterns
+h [nick] Grants half-operator (%) status to [nick]
+i Invite only. Users need to be invited (with /INVITE) to join the channel
+k [key] Sets a password required to JOIN a channel
+l [limit] Limits the maximum number of people on a channel
+m Moderated channel. Users need to be +v (voice) or higher to talk
+n No external messages. The user must be in the channel to talk
+o [nick] Grants operator (@) status to [nick]
+p Private. Hides the channel from a user's WHOIS reply
+s Secret. Same as private, but also hides channel from /LIST
+t Only channel operators & half-operators can change topic
+v Grants voiced (+) status to [nick]

— Jon [June 2008]