Used in a Photographic context, Data refers to either 'Image Data', 'Meta Data', or 'IPTC Data'.

Image Data

Image Data are the pixels that make up the body of a photograph.

Meta Data

Meta Data is the information attached to the image, detailing its authorship, copyright status, keywords, and license status.

IPTC Data 

IPTC Data is information generated by the Camera that is also attached to the image, detailing the Camera Make and Model, lens, and settings used to make the photograph.