A Microsoft file hosting and synchronization service.
So the first I knew of this is when Microsoft deleted some 10,000 photos and videos of my grandchildren on my PC that had been shared by a parent. I thought they had been lost from the OneDrive server. Disgraceful. I found them in my "recycle" bin and restored them to somewhere safe. I've never trusted OneDrive anyway so everything is backed up separately.
I then saw that the shared folder was now just a link to the online version - and was relieved to see that all were still there.
I've now jumped through all the hoops of trying to set up a network connection and/or mapped drive. The best I can get is a mapped drive (with DOS's NET USE) that allows me to see the folder name but not the contents of the folder. So pretty useless. And for another folder shared by someone else I get Access Denied although obviously I'm using the same signin.
But I have found on Reddit at OneDrive Fail a pointer to RaiDrive. The free version does work for me although it has overbearing adverts. It shows all shared folders, regardless of owner, on a single mapped drive letter so will cover any more folders that get deleted off my PC by Microsoft.