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
? You can download a DBD there and get more information.