A Microsoft file hosting and synchronization service.
You could have asked instead of just deleting everything so I have to download all of my files again. So angry at Microsoft over this. Returning my files to local has been a gigantic pain in the rear end. I have far better things to do than to painstakingly select each folder and choose "always keep on this device". The proposed solution of doing this on the root doesn't work. If I select this option for more than a couple of folders, I am greeted with a Finder error citing "cannot communicate with the helper" (below). I've been working on this for hours and it appears I have several hours to go. Already provided feedback via the option on OneDrive. Bad form, Microsoft.
Also now finding that some files refuse to download or open correctly in the default application. Take for example this message I receive several minutes after attempting to open a PDF in Preview: