Usage: WHO <#channel|nick|mask>
When a WHO is done on a channel you are in, you will get a list of the nicks, masks, and "real names" of everyone in the channel and their status in the channel. There will be a letter after their nick, either a 'H' for "here" or a 'G' for "gone". If there is a 'G' it means they set themselves AWAY. A '%' sign means that the person has UMODE +i set.
When a WHO is done on a channel while outside of the channel, only people who are not +i will show, and if the channel is secret or private, then no names will show.
When a WHO is done on a nick, you will get the info only for the user, operators can specify wildcard masks to list users from the entire network.
/who #webchat/who *.t-dialin.net
A who is a good way to get a list of people and their masks. The who command for a channel is much like the names command except that it gives more information. WHO, when used on a nick, is much like WHOIS except that it gives less information.
Sometimes you want to see nicks and their masks. For example, if you wanted to see if there are clones in the channel then WHO would be the best tool. Or it can be an easy way to see how many people are actually present. Although in large channels a NAMES is better for most purposes, because it is shorter and a WHO may fill up your screen.
Note: the special case "/WHO 0 o" will show all opers on the network, see also OPER to list all opers on the server you are on.