The term for this is called "metadata". It can be a myriad of information that a program writer sees fit to add to a particular file. One of the main reasons this occurs is due to file protection. A photo, for example, may be something you created, thus, your information (i.e. your name, other computer elements such as what program you used to make it, date it was created) assists in copyright, and if distributed allows for knowing what you used.
This is typical in Microsoft Word - which adds metadata such as name, business name, and the fact that you used Word. Therefore, if this file is distributed - the metadata states it's yours and that it's an Office file. What's convenient about Word is that there is an option to remove personal information from file.
If you want to remove metadata from a file, then do a search for "Metadata Removal Tools" and you'll be all set.
