Not sure about Ghost, but dd stands for "disk dump". It dumps entire disks to a file.
To use dd, you need to specify input and output.
dd if=<input> of=<output>
is the basic format
<input> is the source. It might be a drive, in that case, it will be /dev/something. all devices in linux have a /dev entry.
<output> is the file. so if you're dumping /dev/sda1 to file named backup.img, it would be /path/backup.img as an example.
Since I only use dd for small operations, I'm not sure how the restore process would work. I imagine it would be the opposite like:
dd if=/path/backup.img of=/dev/sda1
not positive though.