Channels are dedicated to specific topics. In each channel, a list is visible on the right showing all players who have that channel open (This does not work for channels with too many players in it.).
You can choose which channels you have open by clicking on the My Channels icon in the Main Toolbar. A dialog will open and you can scroll down as you click on each channel you wish to join. A check will appear in the box. To remove a channel, click the folder and uncheck the box for that channel you wish removed.
Some of these channels are restricted to Groups. You have to be a member of such a group to enter these channels.
Anything said in a channel can be read by all who have that channel open, and each line of such conversation is called a channel tell. It is necessary to pay attention to what is appropriate for the channel you are talking in. Some channels are reserved for special topics, others for more general conversation. For instance, channel 103 is for religion, but politics is inappropriate there. On the other hand, channel 43 is for chess theory, and neither politics nor religion is appropriate there.
The Channels window
What behavior is allowed in Channels
List of ICC channels with their topics