POP and IMAP are two protocaols lets us to access email from server through mail clients with out the aid(limitation) of a web browser. Both of them server the same purpose but differ little in how they deliver mail and retain mail and status in server and client to client
POP ( Post Office Protocol)
Downloads mail to your local computer , and in this case the changes made of local mail copies will not be sync with the server copy. The server mails will remain untouched
IMAP ( Internet Message Access Protocol)
As the name indicates , access mails from server and sync what ever changed made on mails. This is useful when connecting mail using two clients. the mail box over the remote server will appear identical to both clients accessing.
Which one POP or IMAP
It Depends, for an Administrator when allocates mail to staff POP is recommended as the mail will kept untouched. Where as in cases while more than one person need to access same mail go for IMAP as changes will be sync to client to server then to client