iCloud & iCloud Photos · iCloud Drive basics
What is iCloud Drive on a Mac?
Short answer: iCloud Drive is Apple's built-in file sync service. It appears in Finder's sidebar, keeps your files in iCloud across your devices, and can store them on your Mac or as online-only files that download when opened.
iCloud Drive is Apple's file-storage service built into macOS. It keeps the files you put in it in iCloud and syncs them across your Mac, iPhone, iPad and iCloud.com. On a Mac it appears as iCloud Drive in Finder's sidebar.
Files in iCloud Drive can be downloaded (stored on your Mac) or online-only (stored in iCloud, downloaded when you open them). With Optimize Mac Storage on, macOS moves older files to online-only by itself when space runs low.
iCloud Drive is a sync service: deleting a file from it deletes it everywhere. It's not the same as a backup. Storage Studio works with iCloud Drive's folder on your Mac to make downloaded files online-only and to measure what iCloud uses. See does Storage Studio work with iCloud Drive.