From MIXXnet Wiki
A channel is a virtual space on an IRC server. Users join channels and chat. IRC servers can have many channels. Messages sent to one channel are not seen in another.
Taken from Intro to IRC:
It is not uncommon for an IRC server to have dozens, hundreds or even thousands of chat channels open simultaneously. There are some more or less permanent channels, but others come and go. Although a channel's name usually reflects the general nature of the conversation within, each channel can also have a specific topic. Channel names tend to remain constant, while topics change continuously. For example, in a channel called "PC Users" the topic might be "Windows 95 Bug Fixes" one day and "How to Choose a High Speed Modem" the next day.
You'll notice that all channel names begin with #. One popular and longstanding channel, for instance, is #help. If you decide to wade in, just type: '/JOIN #chat' and voila, you're in. Type in some greetings and you'll see them appear on screen, along with whatever everyone else types.