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).
While some commands may be peculiar to specific IRC client programs, most IRC programs support the following general commands:
/HELP shows general help or help on the given command. This is obviously the most important command and the first one you should investigate.
/LIST lists all current channels.
/JOIN # to join a channel (example: /JOIN #ESP223 means you have joined a channel called ESP223)
/PART to leave a channel; same as LEAVE (example: /PART #ESP223
/QUIT exits your IRC session, (same as BYE and EXIT)
/NICK changes your nickname; you need a nickname to join in the first place. Some systems have character limits (9 usually), and no duplication is allowed. If you try to join with a nickname already in use, you will be asked to change it.
/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. For example, if you sent the message: "Me grins broadly." and your nickname is Bullwinkle, the other users will see: "Bullwinkle grins broadly."
There are many more commands, and some of them are quite complex. Each IRC client will have a listing and an explanation of the commands it supports.