This article is from the LAN Mail Protocols FAQ, by John Wobus firstname.lastname@example.org with numerous contributions by others.
These are rather minimal and are designed to be so. The three are
similar but not enough alike to be interoperable. They are basically
designed to identify the user by username and password, to transfer
the mail from server to desktop computer and to delete the mail
transferred. It is assumed that SMTP will be used to send mail.
Messages can be retrieved individually, but the only information you
can get about a message without transferring it is its length in
bytes-- useful for desktop computers with limited storage.
POP3 has a number of optional extensions including Xtnd Xmit which
allows clients to send mail through the POP3 session rather than using
SMTP. Another extension is APOP which allows RSA MD5 encryption of
passwords passed over the network.
POP3 is now by far the most-used variant of POP, but POP2 may still be
used at a few sites. POP3 has a couple of optional extensions: one to
avoid sending passwords, and one to aid in reading bulletin boards.