A Microsoft file hosting and synchronization service.
I have tried MS' workaround that they tell you about AFTER MS has deleted all your data. If you click the "Always keep on this device' option, it appears to eventually store the file locally. You are now unable to see file or folder sizes and there is a significant delay in loading the supposedly local files. Totally unusable. I have no choice other than to stop using Onedrive. It makes me wonder whether it is time to ditch Office 365 (I have disabled auto renew until I make up my mind).