SMTP is the internet protocol used when sending mail from an end-user client (Netscape Mail, Eudora, etc) for delivery to the local mailserver. The "local" mailserver is then respobsible for forwarding the mail so it will reach the final recipient. SMTP is also used by a mail server (Sendmail, Qmail, MS Exchange) when delivering internet mail to another mail server. If you are on the unix command-line there are other ways of getting your message into the "local mailserver" to start delivery other than your program talking with the server directly via SMTP.

No matter how you send internet mail to someone it will almost certainly take part of the journey over SMTP. Mail generally hops across several mail servers on its path to delivery. E-mail is not designed for immediate delivery. Best efforts will be made to get the message to the recipient in a timely fashion, but anytime a mail-server is under load (handling many messages) delivery times will increase.

POP3 (among others like IMAP) is a protocol used by end-user agents (and some mail forwarding agents, like feetchmail) to retrieve mail from a mailbox.