Show all of the nicknames that are in the specified room. If you are a server operator and no room is specified then it shows the nicks of all the users on the network. If you are not in a room and try to get the names list, +I users will not show up, but if you are in the room then you can get a complete list.
Some clients do not show the users in a room, and so you can use this command to list them after joining the room. Listing all of the users on the network is rarely useful to do, and is limited to opers to prevent normal users from sending annoying messages to people they do not know.