A message, especially one following the common RFC 822 standard, begins with several lines of headers, followed by a blank line, and the body of the message. Most e-mail systems now support the MIME standard which allows the message body to contain "attachments" of different kinds rather than just one block of plain ASCII text. It is conventional for the body to end with a signature.
Headers give the name and electronic mail address of the sender and recipient(s), the time and date when it was sent and a subject. There are many other headers which may get added by different message handling systems during delivery.
The message is "composed" by the sender, usually using a special program - a "Mail User Agent" (MUA). It is then passed to some kind of "Message Transfer Agent" (MTA) - a program which is responsible for either delivering the message locally or passing it to another MTA, often on another host. MTAs on different hosts on a network often communicate using SMTP. The message is eventually delivered to the recipient's mailbox - normally a file on his computer - from where he can read it using a mail reading program (which may or may not be the same MUA as used by the sender).
The form "email" is also common, but is less suggestive of the correct pronunciation and derivation than "e-mail". The word is used as a noun for the concept ("Isn't e-mail great?", "Are you on e-mail?"), a collection of (unread) messages ("I spent all night reading my e-mail"), and as a verb meaning "to send (something in) an e-mail message" ("I'll e-mail you (my report)"). The use of "an e-mail" as a count noun for an e-mail message, and plural "e-mails", is now (2000) also well established despite the fact that "mail" is definitely a mass noun.
Oddly enough, the word "emailed" is actually listed in the Oxford English Dictionary. It means "embossed (with a raised pattern) or arranged in a net work". A use from 1480 is given. The word is derived from French "emmailleure", network. Also, "email" is German for enamel.