Lets say you have a hard drive on /dev/hd1 that you would like to copy to /dev/hd2, you can use dd as follows
dd if=/dev/hd1 of=/dev/hd2 bs=1024
This will create a bit-for-bit copy of the original hard drive.
dd allows you to convert and copy a file with many cool options.
dd - convert and copy a file
Copy a file, converting and formatting according to the options.
force ibs=BYTES and obs=BYTES
convert BYTES bytes at a time
convert the file as per the comma separated keyword list
copy only BLOCKS input blocks
read BYTES bytes at a time
read from FILE instead of stdin
write BYTES bytes at a time
write to FILE instead of stdout
skip BLOCKS obs-sized blocks at start of output
skip BLOCKS ibs-sized blocks at start of input
--help display this help and exit