You can also google it and read the Wikipedia information.
Perhaps not as “hip” as Dropbox or other similar services, but it is the workhorse of deliberate online backup with massively more free storage.
Personally, I think it serves best as a picture archiver. With a current announced size of 7 gigabytes, you can backup an extraordinary amount of photos for free. Once uploaded, you can control whether anyone else sees them, allow zip downloads of entire sets, organize them into nested folders, tag them, etc.
(Since I was an original member of Skydrive, I kept my 25 gigabytes with OneDrive. Extra storage is dirt cheap through Microsoft. Also, Microsoft has a dropbox-like interface now, too.)
Mostly, though, I use it because it is easy to use, free, and has massive storage. It can be used for documents and other files, too – I just don’t use it that way personally.
I gave up trying to tell friends and family about it. Frankly, most people are just too lazy or disinterested to take advantage. They don’t “have time” (whatever that means!) to backup their pictures. When an emergency strikes, they are suddenly without any of their data and the crying begins.
If you don’t have all your favorite pictures backed up, the day will come when you have none – and no means to get them back. Windows OneDrive solves this problem. You don’t have to backup ALL your pictures, just the ones that you identify as treasures.