A Microsoft file hosting and synchronization service.
The problem is there for years. Even when using OneDrive for business. I gave up waiting for the fix. Pity, because integration with Office 365 is really cool.
The problem is the integration is not really cool - it is fundamentally broken in some grievously stupid ways. Let me clarify:
1 Outlook: Outlook used to store its pst files in c:\users<user name>\appdata\local\microsoft\outlook folder. This is an excellent place to store them? Why? Well now MS has outlook store pst files in c:\users<user name>\documents\outlook folder. When OneDrive goes to synchronize your documents it fails because it cannot synchronise any folder where there are pst files in that folder or the sub folder. In order to make a standard office 365 with Onedrive work you have to move your pst files away from the new (and stupid) default location.
2 - OneNote: Microsoft keep installing OneNotes without asking folks. Where do they store the default OneNote files? You guessed it - My documents folder which then causes OneDrive to fail to synchronise as it wont synchronize folders with OneNote files in them or the sub folders.
3 - Search: Its broken. My hard drive is gummed up with gigs and gigs and gigs of useless Windows 10 files WinSXS folder etc so I limited many files like pictures, videos, etc to only be on the cloud or only on my local disk when required. One Drive search doesn't work online. Therefore I cannot search my files.
The list of absolute bollocks continues - e.g. online files over writing local files and vice versa as OneDrive doesn't sync time zones well or the absolutely slow as a dead horse synchronisation. Dropbox, Google Drive etc all synchronise way faster and without the CPU churn OneDrive causes.
It's integration is poor, slow, flakey, unreliable and down right broken. Then try sharing a file with an external person if you are in an organisation. I keep other software on hand to do that.
Totally underwhelmed.