Each IRC network has its own set of channels. To obtain the complete list of channels you type: /list
On certain networks this list might be very huge.
If you are using mIRC, you can limit the output by adding extra parameters to the command:
/list -topic Shows only channels that have a topic set
/list -min <x> Skips all channels with fewer than x users
/list -max <x> Skips all channels with more than x users
/list -wide Displays a list with full channel names and the number of users
A topic, if any, is ignored
/list #channel Shows the list entry for #channel only
/list *word* Shows only list entries containing a certain word in the channel name
Example: /list -min 20 programming