IMPORTANT NOTE : ALL IRC COMMANDS START WITH A "/".

The forward slash is the default command character. Commands on IRC are not case sensitive, and can be abbreviated to their first letters. Anything that does not begin with "/" is assumed to be a message to someone and will be sent to your current channel, or to a person you are chatting with in a private chat (see below).

HELP shows general help or help on the given command.
LIST lists all current channels.
JOIN to join a channel
PART to leave a channel (same as LEAVE)
QUIT exits your IRC session, (same as BYE and EXIT)
NICK changes your nickname
AWAY leaves a message saying you're away or not paying attention
WHOIS displays information about someone
INVITE sends an invitation to another user
KICK gets rid of someone on a channel
TOPIC changes the topic of the channel
ME sends anything about you to a channel or QUERY

/HELP [command] Shows general help or help on the given command.

/LIST [[{flags}] {channel mask}] Lists all current channels.
In the list you will see all channels (see below), except for those that are secret, with their number of users and the topic. The displayed list may be quite long, so you can limit it using flags. "/LIST -MIN n" for instance removes channels with less than 'n' users from the output.

/JOIN {#channel} Sets your current channel to the supplied channel.
Upon entering a channel, you are given useful details about it: a list of users talking in that channel, channel mode settings and the topic... Joining a channel does not cause you to leave your previous channel and you can normally join as many channels as your connection can handle or that the IRC server allows.
/JOIN #windows
*** Now talking in #windows

/PART [#channel] Makes you leave a channel. (same as LEAVE)
/PART #windows
*** You have left #windows

/QUIT [reason] Exits your IRC session. (Also BYE and EXIT.)
If a reason is supplied, it is displayed to other people on your channels.
/QUIT Lunch Time!

/NICK {nickname} Changes your nickname to whatever you like.
Everyone who wants to talk to you sees this name. Nicknames are limited to 9 characters max. If your intended nickname clashes with someone else's as you enter IRC, you will not be able to enter until you change it to something else. Duplicate nicknames are not allowed; this is enforced by the IRC servers. Under some circumstances, two individuals may temporarily have the same nick but once discovered, both of them will be killed; a nick collision kill.
/NICK Goru
*** Newbie is now known as Goru

/AWAY [away message] Sets your status as away with some info.
Sets a message explaining that you are not currently paying attention to IRC. Whenever someone sends you a MSG or does a WHOIS on you, they automatically see whatever message you have set. Using AWAY with no parameters marks you as no longer being away.
/AWAY Gone to get a cup of coffee.
*** You have been marked as being away
/AWAY
*** You are no longer marked as being away

/WHOIS {nickname} Shows information about someone.
/WHOIS Guru
*** Guru is master@some.network.net (Nuclear free)
*** on channels: @#Windows @#Windows95 #mIRC
*** on via server irc.server.net (The best server)
*** Guru is away (making dinner)
/WHOIS Newbie
*** Newbie: No such nickname

/INVITE {nickname} {#channel} Invites another user to a channel you are on.
If you want a friend to join your channel you can invite him. He will see a message such as ***Guru invites you to #channel. This is required if your channel is 'invite only'.
/INVITE Friend #windows
*** Inviting Friend to #windows
If you receive an INVITE message, you can type "/JOIN {#channel}".

/KICK {#channel} {nickname} Kicks a user off a given channel.
Well, you guessed it, if there is a way to invite someone on a channel, there is also the ablility to KICK someone out of it. For example ,if a person is behaving in an offensive manner by annoying people or flooding the channel with unwanted information, they can be forced out of the channel. Only 'channel operators' are privileged to use this command.
/KICK #windows Lamer
*** Lamer has been kicked off channel #windows by Goru

/TOPIC {#channel} {topic for channel} Changes the channel's topic.
Channels have topics, that indicate the current topic of conversation. You can change this topic on a channel with the TOPIC command.
/TOPIC #windows Lets discuss OS/2
*** Goru has changed topic to "Lets discuss OS/2"

/ME {action description} Tells people about what you are doing.
At times, you may want to send a description of what you are doing or how you are feeling or just anything concerning you on the current channel or in a query.
/ME slaps Newbie with a large trout.
* Guru slaps newbie with a large trout.

The same goal can be achieved towards a specific nickname or channel using:
/DESCRIBE {nickname|#channel} {action description}

MSG sends a private message
QUERY starts a private conversation
NOTICE sends a private message
NOTIFY informs you when people logging in or out IRC
IGNORE removes output from specific people

/MSG {nickname|channel} {text} Sends a (private) message to specified nickname or channel.
Besides chatting on IRC Channels you can also have private conversations or queries with other people on IRC. On most clients these conversations will be handled by separate window. You can use the /MSG command to send someone a message that only that person can read. If somebody else sends you a message or that person replies to your message a query window icon will pop up informing you somebody wants to talk to you personally.
/MSG Kreet This message can be read by you only.
*Kreet* This message can be read by you only.

In Kreet's screen an icon will pop up with the message you typed; "This message can be read by you only."

If you cannot wait for a reply for someone to message you to open a private window you can use the query command to force your client to open a private conversation window.

/QUERY {nickname} [test] Starts a private conversation with {nickname} and forces a separate window to open.
This command differs from the MSG command only by the fact that it is used to start a private conversation. All text you type that would normally be sent to your chat partner if you used MSG now displays in an immediately opened private window 'to your chat partner' on your screen and is sent to the other person as well.

/NOTICE {nickname|#channel} {text} Sends a private message to the specified {nickname}or {#channel}.
The NOTICE command is just another way to send messages to other people. But, unlike MSG's, NOTICEs will never open a separate window 'to' the other person. It should be seen as a sort of whispering. It is recommended that robots or other automatons on IRC use notices (contrary to messages) to send information to people. You should never automatically (as by remote events or commands) send a message or notice in response to a notice sent to you.

/NOTIFY [nickname|on|off] Toggles the notify function or adds or removes {nickname} to the notify list.
As you start to meet people on IRC, you will want to add certain nicknames to your notify list such that you will be warned when they sign on or off IRC.
/NOTIFY wug marl
*** Added wug to Notify list
*** Added marl to Notify list
/NOTIFY
*** wug is on IRC
*** resi is on IRC
*** marl is not on IRC
*** kreet is on IRC

/IGNORE [nickname|user@host] Ignore all contact from the specified people.
The day will come when you decide not (never?) to see or hear a specific person on your screen. This can be achieved using the ignore command. If people are flooding channels with useless text or they are otherwise harassing you, a wise response is to ignore those person. Ignore can be set to a nickname or by specifying a user@host format. You can use all kind of wildcards.
/IGNORE looser
*** Added looser to Ignore list
/IGNORE
*** Ignore is ON
*** Ignoring: *.*@*.unicomp.net *!*ap@159.148.109.88 *!*fishy@*.interaccess.com looser
/IGNORE looser
*** Removed looser from Ignore list