A Microsoft file hosting and synchronization service.
If OneDrive online has fewer files in the Camera Roll folder than in the Camera Roll on your PC, you might not be syncing your PC's Pictures folder, which contains the Camera Roll folder, to OneDrive.
Go to Manage Backup on your PC and check whether your Pictures folder is enabled for backup. I suggest you toggle ON the PC Pictures folder. If you lack space on your PC, you can always use the Files On-Demand-Free up Space feature.
See the video here: https://support.microsoft.com/en-US/onedrive/back-up-your-folders-with-onedrive