To use IRC, you choose a "nickname" and connect to an IRC server with an IRC client. Once connected, you join a "channel" and "talk" by typing your message. Once the message is sent (usually by pressing return), it goes out to everyone in the channel. You can also send private messages to people.
A server, as stated before, it's a machine that you connect to to use IRC. Which server you connect to will control what "channels" you will have access to, and what rules you have to operate under. Since a machine can only handle so many requests, there are servers set up around the world. Most of the servers are networked together to "relay" traffic back and forth so that it seems like one big happy network. Unfortunately, not all IRC servers talk to one another. There are three major IRC networks (EFnet, Undernet, and DALnet) and several smaller ones. None of the networks talk to one another, though you can use any IRC client to get on any of the IRC networks.
The name "channel" harkens back to CB radio. In CB, in order to talk to someone, you had to tune your radio to a channel to "talk" to others. In IRC parlance, you "join" a channel. But unlike the non-descript channel "numbers" of CB radio, the channels in IRC have names. There are two types of channels: Local and Network. Local channels begin with an ampersand. These are 'local' with respect to the server you connect to. Channels of this sort will only appear on a particular server. Network channels begin with a pound sign. These channels are available on all servers connected to the same network. Almost all channels you will see are network-channels. To join a channel, you type '/join channel-name' into your IRC client.
On IRC, you don't really "talk" in the conventional sense. IRC is text-only, so you have to convey your message with typed words. And unlike CB-radio, everyone can be talking at once. When you type stuff into your IRC client, it is stored in a buffer. Once you press return, the buffer gets sent off to the channel. The server you are connected to "relays" this message to the other servers.
When the connection between the servers is slow, text will not appear to others on the same channel quickly. When that delay is significant (usually more than 30 seconds or so), you are eperiencing what is known as "Lag." Though you might think you've sent it off because it appears on the channel on your screen, it hasn't been relayed to them yet and thus hasn't shown up on their screen.
Each person that talks on IRC has what is called a "Nickname", or often just called "Nick." This is a 9 character identifier that uniquely identifies you. When you send out a message on a channel, it will be prefixed by your nick. When others send messages to a channel, their messages will be prefixed by their nick. Unfortunately, nicks are given out on a first come, first served basis. During our RadioNet broadcast, I usually use the nick "PhoneBoy". At other times, when I'm on the Undernet, I use the nick "JungleMan". Anyone can use PhoneBoy or JungleMan if they log onto IRC before I do, unless you use DALnet or some of the smaller IRC networks that provide nickname protection services. These services make it so you and only you will be able to use your nick as long as you use it on a regular basis and give it a password that nobody knows.
Here's an example of a session on IRC where several people are talking to each other:
<Kanda-Mi> zombie32: We had someone on earlier with that nick.
We've sort of developed this mild fascination with finding out who it was.
<zOmbie32> ahhhh i got ya :)
<Pal4U> Ducky: I have a hand-me-down recipe for a teriaki marinade.
It has soy sauce, sugar and green onions.
<zOmbie32> sorry i cant help
<Pal4U> Kanda: did you do a whois on him/her/it?
<Kanda-Mi> Pal4u: Yup! Didn't recognize anything.
<Kanda-Mi> zombie32: So what's up?
<Kanda-Mi> zombie32: Hey, are you that Zombie guy from usenet?
<zOmbie32> 1st time looking at this irc....was always using
dalnet
<zOmbie32> its pretty big
<zOmbie32> im not large myself....my wife is a bbw (love bbw's)
(g)
<Kanda-Mi> <hehe> My wife is a BBW too, but I'm also
pretty big myself I guess.
<zOmbie32> is the Mi for michigan kanda?
<Kanda-Mi> No, "Kanda" is 'new' and "Mi" is 'me' in the language
of the Dogon tribe. Hence, "new me".
<zOmbie32> hmmmm...ok :)
<Pal4U> Kanda: Mi=me? Wow, a homophone!
<Kanda-Mi> Seemed apropos at the time -- I was looking for a
new nick. :)
<Kanda-Mi> Pal4U: <hehe> Parallel linguistic evolution perhaps?
<Cosette> or the americanized version, "kinda me" hehe
<zOmbie32> well i from michigan...thats why i asked :)
<Kanda-Mi> Cosette: <hehe>
Earlier I mentioned private messages. A private message is one you address to a particular nick and it only appears on that person's screen. Typically, you type /msg nickname Message (press return). On some IRC clients, this will appear in a different window. On others, it will list the nick with stars around it.
There's a lot of other things you can do in IRC (write scripts, use 'DCC' to send files back and forth, do things to the channel you are on, 'emote' things), but this should be enough to get you started using IRC.
For more info on IRC and the various nets: