E-mail is part of the standard TCP/IP set of protocols. Sending messages is typically done by SMTP (Simple Mail Transfer Protocol) and receiving messages is handled by POP3 (Post Office Protocol 3), or IMAP (Internet Message Access Protocol). IMAP is the newer protocol, allowing you to view and sort messages on the mail server, without downloading them to your hard drive.
Though e-mail was originally developed for sending simple text messages, it has become more robust in the last few years. Now, HTML-based e-mail can use the same code as Web pages to incorporate formatted text, colors, and images into the message. Also, documents can be attached to e-mail messages, allowing files to be transfered via the e-mail protocol. However, since e-mail was not originally designed to handle large file transfers, transferring large documents (over 3 MB, for example) is not allowed by most mail servers. So remember to keep your attachments small!