What exactly is "IRC"?
I've read about it on Wikipedia, but I've never seen people using it (or if I have, I didn't recognize it). The only kind of "chat" I know about are those of AIM, Yahoo! Messenger, etc...
i.e. What is IRC typically used for, and what are its advantages compared to other (more "normal") chat services?
It's an old fashioned chat system, that's very useful for group chat - and many Linux distros use it for informal support. Its strength and weakness is its simplicity - It pretty much won't do anything other than text by default, though there's ways to send files through it as well.
The advantages would be that it works on almost any net-connected system (even when there's no client - there's decent web based front ends), the architecture is fairly simple, it's trivial to run your own server and it's plain text. Authentication is optional, and mostly needed to 'reserve' your nickname or do admin related tasks. It's perfect if you want a service that's anonymous and may have new users pop in at various times.
The disadvantages are by default it's plain text and unless it's a SSL enabled server all your passwords and everything you say can be sniffed by anyone between you and the server.
It's how hackers talk when they don't want to be overheard. It's a pretty primitive chat program. Think of it like shipping channels in the ocean: You can't see them until a boat cuts through the water leaving a wake. If two boats meet in the middle of the ocean to swap illegal drugs, you have to catch them in realtime, otherwise there's no evidence of a meeting left behind.
IRC (Internet Relay Chat) is a real-time multi-user messaging system. Users connect to an IRC server and join one or multiple channels or enter into one-on-one chats with individual users. Users type in messages (up to a few hundred characters long, I'm not sure what the actual limits are) and send them to the channel they are in. Other people in that channel then receive the messages that everyone else has sent. The server is a central point of contact for the channel and serves as a relay for the messages from each user, thus the name. Additionally, servers can be chained together, relaying their traffic back and forth. Typically, an IRC client will render the conversations in a channel as an upward scrolling list of messages in chronological order, with each message on one or several lines (depending on length) and prefixed with some username identifier and perhaps other information (such as a time stamp) depending on the individual configuration of the client.
There are many other real-time communications systems, but IRC has a few key advantages that keep it relevant even today. IRC is decentralized. There is no single company controlling IRC (in contrast to gchat or facebook chat or AIM, etc.) Anyone can set up their own server and use it however they like.
IRC's channel paradigm is very flexible and allows for real-time discussion with up to hundreds of people at once. Most other chat systems don't have similar capabilities. And though an IRC channel with hundreds of active participants can be difficult or impossible to follow, the general case tends to be that only a fraction of the people in a channel are actively chatting at any given time, with the rest either idle and not participating or merely reading.
IRC has the concept of access levels and complex channel modes. For example, users may be banned from a channel based on their IP address, or a channel may be secret (not showing up in the list of channels on the server) or private and require an invitation or a secret code to enter. A user in a channel may be an operator or administrator (capable of banning people, changing channel modes, etc.) Non-operator users in a "moderated" channel are not able to speak unless they've been given voice by an operator. These options render difficult chat situations (e.g. hundreds of people including folks who wish to be disruptive) manageable.
A typical IRC channel tends to be attached to an event or a community in some way. For example, UStream (real-time video streaming) uses IRC for real-time communication between the streamer and the viewers. The advantages listed above in terms of the ability of an IRC server to be controlled by a specific group and access and control of an IRC channel to be controlled by specific users can help the channel owners maintain as high a level of signal to noise as they desire (by kicking/banning unwanted users, using an invitation only system, moderating a channel, etc.)
IRC is often used by friends or communities as a virtual place to hang out. There's not necessarily any grand purpose to a channel other than for people of similar interests or a group of friends to talk to each other.
IRC can also be used for other purposes, especially if specialized client add-ons or fully autonomous bot-clients are used, such as file-sharing, botnet control, or gaming where messages contain commands instead of just chatter.
It's a oldschool way to communicate.
In the begining of the "internet era" it was popular because (besides comunicating with people you know) you were able to get to know new people just by accident, knowing them only from subject of the channel they're connected and from their nickname.
First you have to install IRC client, then connect to server and then to the room (or channel). You were able to talk in 'common' (open) channel where everyone talked or in private with somebody.
OP (from Operator) of this room was able to "kick" aggresive users, trolls etc, OP were able to ban users from room for amount of time (they couldn't reconnect to the room) if they were very annoing or (for example) for spaming links.
It was so popular too becouse anyone with some skills could "listen" to lots of conversations :)
I myself have been one of those people who never understood the purpose of IRC, until recently where I found one reason I could actually use it for.
Whenever you have one of those questions which doesn't quite deserve it's own thread, since you know the answer will be short, IRC is a great way to just briefly state your question and usually get a reply faster than you could by creating a forum-thread.
Also, many forums have lots of rules and they tend to be more formal. So thread-creators often pick their words carefully, put lots of work into their question since it will most likely stay there linked to their identity for a long long time for anyone to see.
From my experience, IRC is usually more casual and less strict, where asking a quick question without providing too many details is not something that will immediately cause negative reactions from people.
After learning about IRC, I was surprised how many websites/projects/communities actually have one which are active. Whenever your in doubt, just Google the name +IRC. I got an iphone a few days ago, had a question in regards to jailbreaking, so I just googled "jailbreak irc", and instantly got in touch with people who had massive experience and could help me out :)
IRC can also be used to open a channel to a single recipient, thus ensuring a "private chat." It's been so long since I used the program though that I forget the command. File transfers through IRC are also very fast.
