A Microsoft file hosting and synchronization service.
And it completely broke my backup scheme, Acronis Cyber Protect Home Office. The update to OneDrive moved my local files to a subdirectory of ~username/Library/CloudStorage, which contains either actual files or references to files on the cloud. Acronis can't back up cloud files, so the backups always failed.
Until I downloaded everything by doing Finder copies of everything on OneDrive and then deleting OneDrive from the computer.
This cost me a bundle of time and $$, as I'd assumed my 16 GB WD backup NAS was hosed and replaced it.
Slick move, Microsoft. It's stuff like this that makes me a Mac user.