Blocks: When one starts to carve with only a chunk of wood that is unprepared, they are using/carving a block.
Cutout Blanks: Cutout Blanks, on the other hand, are a piece of wood that has had some of the excess wood removed; usually be a saw. Most of the time the cutout blank is a flat one or two dimensional piece of wood that a scroll saw or band saw has cut into the shape to be carved.
Rough Out: A rough out is a piece of wood that has had more excess wood removed; and more resembles the final carving. A rough out lacks the finished detail. Rough out are usually done by a machine that follows a program sample from another carver.
There is obviously a difference in the reasons you may use these as a start to a carving. These will be discussed in the next post.