This is a mail protocol that generally allows clients to download emails from the Inbox of their account. Normally POP3 clients will download complete messages and remove them from the server when they have done so. Most email clients though provide options to "Leave messages on the server" allowing messages to be retained on the server.

POP3 email clients are not notified of new message arrival. They poll for new messages to download, and most clients have a "check mail" option. 

The POP service also does not support message flags. All flag changes made to messages within the email client will be local to the email client and not pushed to the server.

Subscribing to folders are also not supported by the POP service, as the POP protocol only supports the Inbox. Therefore any folders created within the email client are local to the client and will not be pushed to the server. Vice versa, any folders that have been created that reside on server cannot be subscribed to.

IMAP4 is a mail protocol that allows users to be disconnected from the main messaging system and still be able to process mail. Users can store messages on a local machine or on a server. A client application would usually cache any messages downloaded, for quick access.

IMAP has distinct advantages over POP because it allows management of multiple folders on the server. Mail can be accessed from different machines, as the mail is left on the server. Clients can send various commands to get or set specific message details. This is useful when operating over slow speed dial-up connections. It also means that all messages are left on the server until the client explicitly decides to mark them for deletion and selects to purge the marked items.

The benefit of accessing the mailboxes mail store on the server is that it allows multiple clients to work on the same mailbox. Through the use of flags (which are stored on server) it allows email clients connected to the same mailbox to see message changes. I.e. read/unread state, flag etc. If one connection deletes an email, other clients connected to the folder will also see the message be deleted.

IMAP4 can break up and download specific parts of a message. This means that instead of having to wait for an email with attachments to download, it is possible to select only the text portion to download, and leave the attachments on the server. IMAP4 supports notifications from the server and where the email client will receive notifications of new message arrivals.