To see what exactly dd is, you should go to the man page.
It essentially reads and writes files(which includes devices). If you want to duplicate a disk, you may type
dd if=/dev/hdc of=/dev/hdd
where hdc is the original, and hdd is the output device, both are of identical size, they are NOT mounted, and there WILL be no errors. It doesn't matter what was in the hdd before, it will be destroyed.
Take care! "dd" doesn't forgive! it can be dangerous