IMAP is, er, I am not really sure but a database, or a database protocol, or a database server of user passwords.
The long and short is that instead of having users manually create their own passwords, one uses IMAP (and LDAP etc.) which is a database which checks to see if a particular username password combination is kosher.
My university has an IMAP server, with an internet address. Moodle can be configured to talk to that server, so that when people enter their username password combination, instead of looking in the moodle database, moodle sends a message to the IMAP server to ask if such a username, password combination exists.
Many universities already have one of these authentication devices.
The upshot is that students do not have to apply for and remember a different password, they only need to use the one that has been created for them and is stored in the central authentication server.
It is surprisingly easy to set up. And these external methods of indentification are almost essential as far as I am concerned. If I had to deal with all the students that could not remember their passwords, all the students that entered an incorrect email addresses when applying for a password, all the students that do not even know their email address, then I would go barmy.
But, praise Moodle, all I have to do is enter the address of our password database server, and then students can use the username/password that they usually use when logging in to the university computers. If they forget their password, then they can ask the central university administrators for a new password.
> Why and when would you choose one over the other?
I would always choose external authentication over internal authentication if such exists.
> How does it look?
It looks like a url typed into the GUI for external authentication from the admin menu. I can send you the screen shot if necessary but you can see it by pressing authentication buttons in your moodle.
> Any thing or anywhere I can read about it?
I don't know where you can read about it. sorry.