Well, a normal watermark is just simply like this: (the COMSTOCK images logo)
It can be in text form or logo, over the picture to 'mark' the picture so that it discourages people from taking the photo as they have to use photoshop to edit the watermark if they want to steal the photo..which is not impossible to do..but extremely tedious.
Another kind of watermark is the 'Digital Watermark' it is a coded thingy that is attached to the picture.. i can't remember how it works. It's just some code thingy
I think someone else can give a better explanation..but this is the geist of it .. hope it helps :lovegrin: