Linux command: dd

dd

It's used to manage images (create / dump)
To dump an image to a device (WARNING, IF YOU ENTER THE WRONG DEVICE IT WILL BE OVERRITEN)

dd bs=4M if="/home/linux/2013-02-09-wheezy-raspbian.img" of=/dev/mmcblk0

To create an image from a CD:

dd if=/dev/cdrom of=/tmp/cdimg1.iso

(*) Note: The cdrom must not be mounted.
You can check the image with md5sum.
To see progres:

dd status=progress if=/dev/cdrom of=/tmp/cdimg1.iso

Create compressed image

sudo dd if=/dev/sda conv=sync,noerror bs=64K | gzip -c  > /PATH/TO/DRIVE/backup_image.img.gz

Restore compressed image

gunzip -c /PATH/TO/DRIVE/backup_image.img.gz | dd of=/dev/sda