KMail allows you to access your electronic mail in various ways. The access type largely depends on your Internet service provider (ISP) or mail server. The most important access types are POP3, IMAP, and dIMAP:
POP3 (post office protocol version 3) is a standardized method (protocol) for the delivery of e-mail. A POP3 server stores the messages until the user retrieves them with a POP3 client (in this case, KMail). The user may or may not be asked to authenticate with a name and a password. The drawback of this protocol is that it transmits passwords as plain text. To remedy this, alternative protocols have been developed, namely APOP and KPOP. These transmit passwords in encrypted form. Most ISPs use POP3. For a more detailed explanation of the protocol, see http://en.wikipedia.org/wiki/Post_Office_Protocol.
Unlike a POP3 server, an IMAP mail server stores the messages and allows you to access them from it. They are only downloaded to your client host as needed. This offers a number of advantages. One of them is that the same messages are available from several client hosts (your machine at work and your laptop used on the road, for example). Another one is that mailboxes can be shared among different users. For a more detailed explanation of the protocol, see http://en.wikipedia.org/wiki/IMAP.
dIMAP offers the same possibilities as IMAP with the additional advantage that messages can be edited offline. This is a big advantage for those situations where a permanent connection to the server is not available as when using a laptop. With dIMAP, messages edited offline are synched with the server as soon as the connection is reestablished.