This is a comparison of online backup services.
Online backup is a special kind of online storage service; however, various products that are designed for file storage may not have features or characteristics that others designed for backup have.
A browser-only online storage service is usually not considered a valid online backup service.
Online folder sync services can be used for backup purposes.
If a file is accidentally locally corrupted or deleted, it depends on the versioning features of a Folder Sync service, whether this file will still be retrievable.