When a who is done on a room you are in, you will get a list of the nicks, masks, and "real names" of everyone in the room and their status in the room. 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 is +I, invisible. When a who is done outside the room only people who are not +I will show, and if the room is secret or private no names will show. When a who is done on a nick, you will see the person's name, mask, and "real name".
A who is a good way to get a list of people and their masks. The who command for a room 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 room 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 rooms a /names is better for most purposes because it is shorter and a /who may fill up your screen.