POP and IMAP are communication protocols that download your e-mails to your personal computer or other devices.

POP3
It provides basic downloading and deleting actions. E-mail service providers who use POP usually take actions as follows; connect to the server, get all the messages, keep those messages on the user’s device and delete them from the server, disconnect to server. Therefore, when mails are also deleted from the device, they are no longer available. However, some e-mail service providers may provide an option to archive those messages in a new folder. POP3 supports only one user at a time. In fact, if you connect to your account from a second device then it disconnects the first one.

IMAP
When several devices are connected to a mail service provider at the same time, IMAP protocol prevent some possible problems with these devices to occur. Unlikely to POP, IMAP provides bidirectional communication. Thus, some possible problems are inhibited that may occur while connecting on web, synchronizing from mobile devices, getting e-mails from computer. The most useful function of IMAP is that when you move a mail to a new folder then it updates all the connected devices. All the modifications are updated from all connected devices synchronously.