POP3 (Post Office Protocol) and IMAP (Internet Message Access Protocol) are email protocols that you can pick when configuring your email account in an email application (e.g. MS Outlook, Windows Live Mail or others) on a computer or mobile devices or tablets. You can arrive at a stage during the setup procedure where you have to decide between POP3 or IMAP setup.
POP3 downloads all email content into your email app. You can access stored emails even when you’re working offline and store new emails in the OUTBOX ready for delivery when you’re back online. Users can opt to delete the email from the server once it is downloaded to save hosting space or configure to leave a copy of the downloaded email on the mail server for any number of days.
IMAP protocol synchronizes the content of emails between your email application and the mail server, your email application and the mail server, your email application must still be online for you to access the status of your mail. Email content on all devices between your email application and the webmail is the same, as both of them synchronized their mail server content.