POP vs IMAP - the differences, simplified
POP retreives emails, it can either leave a copy on the server or mark the emails for deletion immediately or after a number of days. Emails are downloaded and stored locally, the sent items are not uploaded to the server and will only exist on the device you used to send those emails. POP is good if you have only one device and you want to wipe the emails from the server and never worry about exceeding your storage quota.
IMAP synchronises emails with the server, it allows you to see read/unread emails and sent items across multiple devices (laptop/desktop/mobile phone/tablet). Because all emails are stored on the server, it means you do need to manage your mailbox to stay within storage quotas; by deleting unwanted emails and archiving anything crucial when you approach the mailbox limit.
POP vs IMAP - in more detail
POP is a very simple protocol, making implementation easier
POP mail moves the message from the email server onto your local computer, although there is usually an option to leave the messages on the email server as well.
IMAP leaves the message on the email server, simply downloading a local copy.
POP treats the mailbox as one store, and has no concept of folders
An IMAP client performs complex queries, asking the server for headers, or the bodies of specified messages, or to search for messages meeting certain criteria.
When POP retrieves a message, it receives all parts of it, whereas the IMAP4 protocol allows clients to retrieve any of the individual MIME parts separately - for example retrieving the plain text without retrieving attached files.
IMAP supports flags on the server to keep track of message state: for example, whether or not the message has been read, replied to, or deleted.
IMAP is designed to permit manipulation of remote mailboxes as if they were local. Depending on the IMAP client implementation and the mail architecture desired by the system manager, the user may save messages directly on the client machine,or save them on the server, or be given the choice of doing either.
The POP protocol requires the currently connected client to be the only client connected to the mailbox. In contrast, the IMAP protocol specifically allows simultaneous access by multiple clients and provides mechanisms for clients to detect changes made to the mailbox by other, concurrently connected, clients.
