POP / POP3 - With Post Office Protocol version 3 (POP3) your mail is saved for you in your mail box on the mail server. When you read your mail, all of it is immediately downloaded to your computer/device and no longer maintained on the mail server. For example, if you normally access your emails through an email client on your phone, the emails you view will be downloaded to this specific device. Therefore, if you then view the same inbox on a different device's email client, you may not see the same inbox (since the email was already downloaded to a previous device).
See the KB article "What is the difference between IMAP and POP email accounts?"