Watermarking is a method by which a copyright statement, or any visible mark, can be left on a photograph, piece of art or any online documentation. The watermark is generally used to state the copyright claim of the piece and is accompanied with the usual © symbol and a copyright claim by the author and a date of publication.
Many watermarks are designed in a way to inhibit misuse of said artworks, and can come in a variety of methods and styles. To deter from visually obliterating the image many watermarks tend to be made semi-transparent in order to see detailing behind. The best approach to the watermarking placement is to place the mark in a place that is close to a focus point on the image or across a point of interest. That way, any attempt to remove the watermark will be futile.