What is the difference between POP3 and IMAP?

POP3 (Post Office Protocol 3) is an Internet protocol used to download e-mail messages from a server to a computer using an e-mail client (such as Outlook).
Its primary advantages are that it is widely available,
and e-mail messages are not retained on the server so the maximum number of messages is limited only by the size of your computer hard drive;
its primary disadvantage is that it becomes difficult to use when more than one computer or device needs to access the same e-mail account.

Unlike POP, the IMAP e-mail protocol automatically retains e-mail messages on the server and instead sends
copies of the messages to as many computers and devices as you want and synchronizes all of them.
Its primary advantage is the multiple computers and devices can access the same e-mail account without any conflict;
its disadvantage is that space is limited by the server (or your allotment therein).