What the difference watermarks

What is the difference between (semi-)fragile and robust watermarks?

The aims of such photo watermarks are completely different: A (semi-)fragile watermark is a mark which is (highly) sensitive to a modification of the stego-medium. A fragile watermarking software scheme should be able to detect any change in the signal and identify where it has taken place and possibly what the signal was before modification. It serves at proving the authenticity of a document. On the opposite, a robust photo watermark should be stuck to the document it has been embedded in, in such a way that any signal transform of reasonable strength cannot remove the watermark. Hence a pirate willing to remove the watermark will not succeed unless they debase the document too much to be of commercial interest. The latter form is the very challenging and attracts most research.