It's the type of mail server, or more specifically, the protocol used between the mail
client (e.g. Outlook Express) and the mail server.

POP3 is the oldest and most widely used.  It works reasonably well for one person using
one PC to access their mail account.  Mail is managed on the PC in the mail client.  It
has very limited (and awkward) support for sharing a single server mail account across
multiple PCs or multiple people.

IMAP is a newer protocol and not as widely supported as POP3.  Some mail servers will give
you the choice POP3 or IMAP access.  With IMAP, mail is managed on the server.  It is
designed or overcome the limitations of POP3 when a single mail account is accessed by
multiple people or from multiple PCs.  For more information on IMAP, see:
http://www.imap.org/

HTTP is an invention of Microsoft.  It is used with Microsoft's Hotmail and MSN mail.  It
is supported by newer versions of Outlook Express and Outlook mail client.  I'm not aware
of any mail services other than Microsoft that support it.  Its functionality is similar
to IMAP.

http://www.imap.org/imap.vs.pop.brief.html
http://www.sendmail.org/email-explained.html

When's your homework due?