Guilds are what you'd call clubs that people can join.
The proper way of starting a guild is asking either Karn or one of the Deputies. Every guild has one Guildmaster that can control who may join.
A person can be a member of only one guild.
Here's a small list of all possible guild commands:
guildapply, guildaccept, guildreject, guilddescription, guildurl, guildtitle, guilddetails, guild, guildleave, guildmessage, guildaddrank, guilddelrank, guildassignrank, guildremove, create guild.
To get more information on these commands, type help command, for example, help guildaddrank or help guildassignrank.