Others may work but haven't been tested - you are on your own. Both POP3 and IMAP are communications protocols used to retrieve e-mail from a remote server, without having to log directly onto the server and know the command language of the operating system. It allows the use of local clients with better interfaces. The client then gets a list of all e-mail stored on the server, and downloads them to the local computer. Once done, all the messages are removed from the server, and the connection is closed.
Messages are kept on the server - the client only downloads the list of messages.
When the user requests to read a particular message, then the contents of that message are downloaded. When the user deletes a message, it is deleted from the server - all the other message remain. Copies are not normally kept on the local system, but it depends on the client. Depends on what software you wish to use. Most support POP3 - fewer support IMAP, but more and more are adding it.