Many don’t have a clue what this is, yet use it everyday. You can find this form of communication on JTV, Ustream, BlogTV, and thousands more not know by the average internet user.
IRC chat allow people to communicate live with one another online. Some chat rooms or channels may contain bots which are programmed to use this form of communication as well. Some chat rooms are specific to topics which are set by the channel owners or their Ops while others are free form and you may chat about anything.
Most services listed above have a built in program to allow chat while watching live or pre-recorded video stream. Another way you can chat in those channels or others services is using programs such as mIRC, xCHAT, ICEchat, etc. Most of these programs are free while others charge a minimal one time fee. There is a lot of information on the internet how to setup and use these programs, they may seem intimidating at first but once you begin to use them, they become a bit addicting to use.
Hope the above information gives you a bit more in-site as to what IRC is and inspire you to try using other ways of communicating with some of the listed services.