dd_rescue, is an an advanced evolution of dd, a command line program that has been ported only for UNIX/Linux. The program uses a complex series of flags to allow the user to image or write data from and to raw image files. Like dcfldd, the program makes an effort to keep the user apprised of the status of the current operation.
ddrescue and dd_rescue are completely different programs which share no development between them. The two projects are not related in any way except that they both attempt to enhance the standard dd tool and coincidentally chose similar names for their new programs.
Here is a common dd_rescue command:
$ dd_rescue /dev/hda myfile.img
One example of this usage would be transfering a disk image over the network using ssh.
dd_rescue /dev/sda1 - | ssh email@example.com "cat - > /remote/destination/file.img"
Another example would be adding compression to an image file on the fly.
dd_rescue /dev/sda1 - | bzip2 > /dir/file.img.bz2
Unlike regular dd, dd_rescue does not use the command line arguments if or of.