One way to (sort of) see the difference is to log in to https://www.icloud.com
Information in iCloud that is not in iCloud Drive includes Mail, Contacts, Calendar, Photos, Notes, Reminders, etc. iCloud Drive is a separate app or section within iCloud.
It is possible to upload photos to iCloud Drive but they won't be in the Photos app at iCloud.com and not integrated with the Photos app on your Mac or iPhone. For that you would want to use iCloud Photo Library: iCloud Photo Library - Apple Support
If you backup any iOS devices (iPhone, iPad, iPod Touch) that is part of iCloud but isn't available by logging in to https://www.icloud.com/
For me, it isn't a case of "one over the other" regarding iCloud and iCloud Drive, it is both.