An outbox is where outgoing e-mail messages are temporarily stored. While you are composing a message, most mail programs automatically save a draft of your message in the outbox. The message is then stored in the outbox until it is successfully sent to the recipient. Once the message has been sent, most e-mail programs move the message to the "Sent" or "Sent Messages" folder. While the terms "Outbox" and "Sent Messages" are often used synonymously, technically they have different meanings.
Unlike the inbox, which is often overflowing with e-mail, the outbox often does not contain any messages. This is because all the messages that have been sent have already been transferred to the Sent Messages folder. You can think of an e-mail outbox much like the outbox at an office. Mail that is to be delivered is temporarily placed in the outbox until the mailman (or the designated office mail guy) picks up the mail and brings it to the post office. However, the messages in an e-mail outbox are typically delivered immediately (unless a connection to the outgoing SMTP mail server is not available). If only it was as easy to keep your inbox clean...