POP (Post Office Protocol) is a protocol invented in 1984 and refined until 1998, which allows simple and easy downloading of messages from an email server to the local computer. Email programs (Outlook, Thunderbird, Zimbra Desktop, etc.) retrieve messages from the server and store them on the computer's hard drive. Immediately after downloading, the messages are deleted from the server, freeing up the occupied space. The POP protocol has the advantage of speed, because messages are accessed locally, but it does not allow access from multiple computers (except through technical artifices, which can lead to message corruption). If you configure two computers to receive messages via the POP protocol, you will be surprised to find part of the messages on one computer and the rest on the other, without having all the messages on the same computer.
IMAP (Internet Message Access Protocol) was originally developed in 1986 as an alternative to POP. The major difference between IMAP and POP is that messages are processed directly on the server. The email client brings a copy of the message to the computer or smart terminal, but all operations (modification, deletion, organization in folders) are performed directly on the IMAP server. Hence the major advantage: using the IMAP protocol, you can access messages from several terminals (office computer, home laptop, portable tablet or smart phone), without worrying about losing important messages. By keeping messages on the server, your disadvantage is that of expensive space (hosted storage is much cheaper than space on the provider's server). However, this disadvantage is blurred by most email programs, which allow archiving or local backup of messages.