It is a command. You can use it easily in a shell script. Check on its man page for more details.
read_hdc.sh: - Read first n sectors from hdc
dd if=/dev/hdc of=$2 bs=512 count=$1
Save it into "read_hdc.sh", do a chmod 755 read_hdc.sh
and run with ./read_hdc.sh 80 backup.img
to copy the first 80 sectors of hdc into a file called "backup.img".
"dd" can be used to copy between virtually any two files, providing you have the access. It is also a common tool for doing floppy images/duplications as an option of many Linux distribution installation.