User Data is simply the "file data" the rest of the data which would be Error Correction, Sync etc. make up the rest of the sector. RAW means copying "everything" user data + all the extra data that is not needed, except for copy protection purposes. For DVD all you can copy is the user data, can't do RAW and get that "extra" data.
Think of a full sector like this:
EDC (Error detection code)
ECC (Error correction code)
User Data (file data - this is all you can copy w/ dvd)
RAW copies "everything" which DVD cannot do. You can only copy the "user data" that's it.
I hope that gives you an idea.