IMAP and POP are the protocols that are used when downloading email from a mail server and most mail clients (such as Outlook, Thunderbird, Windows Mail) will accept either when configuring email accounts.
POP (Post Office Protocol)
All your email will be downloaded to the computer (into the mail client) and deleted from the mail server.
IMAP (Internet Message Access Protocol)
When you read your email, the mail client will store the email on the mail server, manipulate it and read it from the server. The email remains on the server, you can access it via another IMAP mail client and view your mail exactly as it appears in your other IMAP client, which will keep everything in sync.
Note: Most mail clients will cache IMAP email. However, with IMAP email you can't manipulate your mail while offline as it requires an active connection to the server. Also, you do not retain your own local copy, so if you delete your mail account all your emails will be deleted as well.
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