insert email content to db

hi guys,

can u guys give me an idea how actually we can insert something into the database based on the email we received ??

it is something like what this website can do..
http://hades.netsonic.net/support/index.php

when i add a new ticket, i will receive an email (i know how this can be done).. when the time they reply me. their reply will be inserted into database and i will get an email too ( i know how to get this done too).. when i replied by using email, whatever i replied will be inserted into their database directly.. how could that be done ?? did they do that by using php ?? since email doesn't go through any file, why whatever i replied will be stored in their database,, this sounds cool to me.

do u guys mind to give me an idea how to create this kind of system ??

thanks.