I suppose the simpliest way to say it is that the DBD is the driver to the database. If I understand the process correctly, the DBD contains the specifics for the particular database that you are trying to connect to. You need to understand both DBI and your chosen driver. I haven't pursued the "why" as much as the "how". You just need all the correct pieces or the connection isn't going to work. Have you looked at the DBI site? You can download a DBD there and get more information.