What is iCloud Drive?

iCloud includes a variety of services such as iCloud Backup for iPhones and iPads, iCloud Photos, and iCloud Drive (with or without the Desktop and Documents option of iClod Drive).

iCloud Photos syncs photos in the Photos app on Macs and/or iPhones/iPads. iCloud Drive syncs some files on Macs and iPhone and iPads.

See the following for more information on iCloud Drive: Access and manage your files in iCloud Drive - Apple Support

And see this for information on the Desktop and Documents option: Add your Desktop and Documents files to iCloud Drive - Apple Support

When Apple ID is signed in with Mac system preferences , an option comes to set up iCloud Drive Set up iCloud Drive on all your devices - Apple Support , when iCloud Drive is enabled , option comes to turn on desktops and documents box , it stores documents and files created see the details Use iCloud Drive on iCloud.com - Apple Support however iCloud stores photos , reminders , back ups ... the details are mentioned in the article Introduction to iCloud - Apple Support