SMTP

Simple Mail Transfer Protocol- This is the "language" used to send emails.  It uses the TCP/IP
(transmission control protocol /internet protocol) to define the format for sending mail across the internet.  The SMTP servers send the messages throughout the internet to a mail server such as POP3 (see below).

POP3

Post Office Protocol 3- This is the standard mail server used on the internet.  It holds the incoming emails until the person receiving the email logs on.  It is then downloaded to the users computer.  POP3 uses the SMTP to transfer mail messages.
 

All of these different protocols are still confusing to me.  I never realized it was so complicated- I just pushed "send" and it was off- always a miracle when it arrived where it was suppose to go.