IMAP and POP are the protocols or technologies using which you can download messages from mail servers on your computer and access them with the help of mail clients such as Microsoft Outlook, Mozilla Thunderbird etc. The main advantage of this technology is that you can access your emails via a feature-rich browser-independent mail client. In case of POP, you get offline access to old mails too.
Difference between IMAP and POP
IMAP and POP are two different protocols. There are many differences between these two. The main difference is that IMAP(Internet Messaged Access Protocol) always syncs with mail server so that any changes you make in your mail client (Microsoft Outlook, Thunderbird) will instantly appear on your webmail inbox.
On the other hand, in POP(Post Office Protocol), your mail client account and mail server are not synced. It means whatever changes you make to your email account in the mail client will not be transferred to the webmail inbox.