Watermark is simply a copyright notice on any image or document, if you found any website name or company name pasted on any image with reduced opacity then it would be watermark. Watermark is use to protect data copyrights, because this particular data is owned by someone so you can not use it. Watermark is also indicate that do not use this data or image because this image owners have all copyrights reserved.
Watermark Second Examples on Scenery:
Watermarked images usage is prohibited and violate Seo terms and conditions. So you should be aware about that.