Mail Store - where mail is placed. RFC 822 - defines the format of a mail message, including all headers and formats for proper email pieces. Spammers often send incompliant mail messages. MIME - a standard for detailing how to attach files to a text email. These are methods of accessing a mail box remotely.
Pop is the older standard, good for low bandwidth, etc.
Rather limited compared to Imap. Imap allows for folders, stay connected, manipulate messages. A little simpler, uses the mbox format on the back end. Cyrus - A more complicated, but more stable system. Products such as squirrelmail.