The main difference between IM and e-mail is the idea of presence [source: The National Archives]. With e-mail, if you compose a message and send it to your friend, you have no idea if your friend is logged onto his computer, when he'll read your message or when or if he'll respond. With IM, however, the software tells you that your friend is logged in and available to chat. If you send your friend an IM, there's a much greater chance that he'll receive the message immediately and respond quickly.
The presence factor of IM qualifies it as a form of real-time communication, similar to telephone calls or face-to-face conversations. With IM, the other party receives your message almost immediately after you press the send button. From a technical perspective, real-time communications implies that there's a direct, open connection between the two or more parties who are talking. For telephone calls, that connection is supplied by both the wired and wireless telephone network. For IM, the Internet supplies the direct, open connection.
E-mail, on the other hand, is a store and forward technology, meaning that outgoing e-mails are stored on a server and forwarded across the Internet to a destination server (for more details, read How E-mail Works). For the recipient to receive the e-mail, she must log on to her e-mail software or webmail program and retrieve any new messages from the destination server. There's no opportunity for a real-time conversation via e-mail, because there will always be a lag between the time a message is sent and the time that it's received.
There are advantages and disadvantages to both IM and e-mail, which is why most people use some combination of both technologies. The advantage of e-mail is that recipients can read and respond to their messages at their own pace. If you're about to step out to lunch and three new e-mails arrive in your inbox, you don't have to answer them right away. The people who sent the e-mails are aware that it might take as long as a couple of hours or even a day to get a response.
Which brings us to the main disadvantage of e-mail, that you might have to wait a couple of hours or even a day for a response. For this reason, e-mail is usually reserved for messages that fall within a certain window of time sensitivity. If you need an answer by the end of the day, e-mail works fine. But if you need an answer right now, you either need to pick up the phone or try IM.
The chief advantage of IM is immediacy. Plans can be made quickly and answers found faster. Groups of people can enter an IM chat room and collaborate in real time. There's no need to e-mail multiple copies of the same idea to six people and wait for them all to respond, usually in a confusing, overlapping combination of messages.
But the main disadvantage of IM is that it doesn't work unless both parties are present. In many cases, this means that both parties need to be logged onto the same IM program, or at least two programs that work with one another. IM isn't like e-mail, which uses a standard technological protocol for sending and receiving messages. Each IM client -- whether it's AOL Instant Messenger, Yahoo! Instant Messenger, ICQ or Google Chat -- uses a different protocol that may or may not be compatible with the rest.
Much of the advantages and disadvantages of IM and e-mail depend on what you want out of your electronic communication. And what you want depends heavily on who you are. In the next section, we'll look at the popularity of IM and e-mail and who uses them.