Instant messages are messages sent by IM users (like you or me) using IM clients (yahoo messenger,AIM messenger,SKYPE,Google Talk,MSN messenger and lots more).In E-mail, the person might not receive an E-mail message in his inbox as soon as you send it(especially if u use a non-web based Email client like Outlook).But When you use an IM client, your message can be viewed by the recipient INSTANTLY, hence it called an "Instant" messenger.