A Microsoft file hosting and synchronization service.
Despite Microsoft promoting OneDrive as a cloud back up system, OneDrive is not and never was intended to be a back up system. It's an aggressive data capturing system based on Microsoft's attitude of dictating to it's customers - something which appears to be the norm these days.
What the majority of hapless unfortunates who constitute OneDrive's enforced users haven't realised - and this originally included me - is that when OneDrive automatically moves all files to the OneDrive cloud, it is saving those files AS BEING THE ORIGINAL ITEMS. i.e. the originals are NOT stored on your P.C., they are stored in OneDrive - the files on your P.C. are the copies. And there is NO backup. This is the antithesis of a file back-up system. No wonder people are confused given how Microsoft promote this system.
Unfortunately, this arrangement makes it very easy to delete files locally and, unwittingly, have that operation synchronised to the OneDrive cloud which means any files deleted from the local documents folder on your PC will be deleted locally AND from OneDrive (as the ORIGINAL FILES) as it re-syncs. Those files are now GONE. Permanently.
Similarly, if a "master user" (by that I mean a person who has shared a OneDrive file of theirs with another person for work collaboration, or to share family photo's, for example), shares a file with another person from their OneDrive, and that other person deletes a file from the shared OneDrive access, that file will be deleted from OneDrive (as the original), as well as locally on the master users PC if synced. Again, GONE. Permanently.
Once you are aware of this set up, the drawbacks are blindingly obvious. So, WHY, Microsoft?