IRC
IRC refers to the Internet Relay Chat protocol, RFC 1459. (I should point out early that if you did not know what IRC stood for, your experience will not be a pleasant one for quite a few months. IRC was designed in 1988 and hasn't changed much since, so generally inhabitants will not tolerate little boys running around in their channel unwillingly radiating their newfagaity everywhere.)
In common use, however, the term is used to refer to whichever networks and servers one familiarizes oneself which run IRC services. A network is a tree of linked servers which can all share channels, or "rooms," and for which there are one set of administrators and operators. Channel communications are distributed network-wide so that the server you choose to use does not prevent you from talking to any one particular person on that network (with all the usual optimization one might imagine). There are a vast number of separate IRC networks.
Using IRC
You must download client software to connect to any given IRC network. If you are a windows user, you are most likely going to end up using mIRC. Wow. You faggot. Many servers on the more popular networks require you to run a service called ident, which historically was intended to be used by the owner of a computer in order to ensure that connections from his or her computer created by users other than him- or herself could be traced back to that user specifically. Of course, there are no girls on the internet, so forget all that shit I just said about "her". Clients for idiots include their own ident server. mIRC is one of these. Linux server administrators and their teenage faggy-distro-mouse-using-xwindows-crippled cretin counterparts can install one of many "identd" packages to the same end, oidentd likely being the most popular.
#ytmnd / EFNet
The #ytmnd IRC channel can be found at EFnet. As of May 15th, 2007, the channel was globally juped by EFNet staff, which simply means #ytmnd is not open for the public anymore, and attempting to join it will get you banned for EFnet for 3 days. However, if you use an EFnet server such as 'irc.vel.net', 'irc.efnet.nl', or any other server that doesn't support global jupes, you can still join the channel and chat. But don't expect much of it. It consists mostly of large Pumped ASCII such as bigmatix or a giant image that bares a slight resemblance to Leonidas of Spartan fame.
There was also a YTMND-specific IRC server located at irc.ytmnd.com. After being ddosed to oblivion, it was sold to host porn, a conclusion drawn simply from the fact that the domain "irc.ytmnd.com" used to show http://sexyhornylesbians.net at the bottom. Max has also confirmed this. Apparently same thing happened with the radio server. Tough luck, Go watch porn instead.
Alternatively, you can join #ytmnd2. Most of the old #ytmnd regulars hang out in there with the exception of max (he hangs out in fucking #?!, which is normally +m and everyone knows +m sucks) and a couple other stubborn fags. Bear in mind that #ytmnd2 is completely unrelated to #ytmnd and, even more so, YTMND. A significant portion of #ytmnd2ers are penis pumpers, and reside in those channels as well.
Other slightly related channels include #ytmndsoundtrack, #ytmndongs, and #tgubg.
Recommended Use
Join and pay attention a maximum of three times per day, every day. Massage affected area gently until you are banned for pasting ridiculous ASCII art which does not honor the correct control character set for standard clients or have any hard spaces in it. Take with food, just be sure clean your keyboard afterwards, you slobby cheeto-eating motherfucker. If rash, vomiting, laughter, happiness, kickban, moderation, or GTFO FAGGOT persists, get the fuck off of your computer. Disregard that, IRC is pretty much better than every other chat medium known to mankind and it will make you extremely popular on the tubes.