I am trying to add metadata to images, and I want it as key/value pairs. I know about the possibility to add keywords. But they are just a list of words. I would like to be able to add the metadata as pairs, like Product=Apple. Is this possible? If so, what type of metadata would it be (xmp, exif...), and how do I add it (e.g. in Adobe Bridge)
Your best bet would be XMP as it allows for specification of custom "namespaces/schemas", though it is XML-based. Full documentation is on Adobe website. In fact this shows you can do it with free ExifTool.