Please use this tag for any questions about email sending, receiving or composing. Also valued for this tag are questions relating specifically to email-specific networking. Tags like mail.app, outlook might be better for client-specific questions. Also note that major mail services like gmail, icloud, mobileme also have tags.
Email is a method of communication and file transferral. Generally, email is hosted on a server, which is accessed either by a web interface, or an email client application which receives the messages from the server.
Two methods of this message retrieval are POP and IMAP. POP gets new messages and stores them as local files on the computer, and whatever the user does with them is simply local. IMAP maintains a connection with the server, and deleted, moved, and read messages reflect back to the server (which is helpful for keeping multiple devices in sync). Many clients, including Apple's Mail app, support both these methods.