Nutshell: SMTP/IMAP/POP3
In one line: SMTP is for sending mails and dropping it to all relevant inbox server. IMAP is for fetching emails from Email server(inbox) to client(app).
That’s all guys! Had fun talking to you. Enough reading, I’ll try to get info via pictures and flash cards.
Now all these SMTP/IMAP/POP3 are just some popular protocols, there are many alternatives available out there, these 3 are most famous ones.
IMAP and POP3 are used for same purpose. IMAP is more famous now so it’s mostly IMAP vs SMTP and POP3 just comes along in replacement to IMAP.
Ref: http://www.differencebetween.net/technology/protocols-formats/difference-between-smtp-and-imap/