Raw image — image containing all drive's data - an exact copy of drive's surface stored in one or set of files. It includes used as well as unused clusters and sectors.
This means that all existing and even some deleted files would be mirrored to another location. Raw image has usually size larger than Data Image, but gives full picture of data, no one bit of data is missed in this case.
Data Image — image containing the only used drive's clusters. Data Image is applicable for logical drives and partitions only.
When you recover Data Image later on — all unused clusters would be just zeroed. This approach however can save you a space and increase a speed of disk image creation.
Since Data Image operates with term used clusters, it applicable to partition (logical volume) only, because only they have file structure. You wouldn't be able to make a data image of entire disk — only raw image.
