OneDrive Architecture

I'm stumped by OneDrive's performance on my wife's Surface Pro. Is OneDrive only in the cloud or does the term "OneDrive" also apply to a folder or virtual folder on the S Pro's hard drive?

When we login on my wife's OneDrive account at onedrive.live.com, there are no files or photos. But in the File Explorer on the S Pro, all her documents and photos are in OneDrive, in C:\Users\<wife>\OneDrive. So where exactly exactly are the files?

The C drive's Properties window says it is nearly full, but is it? Are the OneDrive files in the cloud or not?

Are you sure on the S Pro, One drive is logged in with her account and that Sync is enabled?
You can check the Sync status by clicking in the Notification area on One Drive Icon.