Online backup services have you install software on you PC that scans your storage for files worthy of backup, encrypts them for security, and sends them up to the cloud—that trendy word that just means powerful, secure, and high-storage-capacity server computers attached to the Internet with fast connections. Once your files are stored on those cloud servers, they're accessible for you to restore to the same PC, should a file go missing. In most cases, the service also lets you access your files from a Web browser or mobile device.
Though there's some overlap, online backup services shouldn't be confused with cloud storage and syncing services like Box, Dropbox, Google Drive, and OneDrive. Those services do store files in the cloud, but they aren't designed to automatically protect all important documents and media files, let alone system files. Their strategy is generally to sync just one folder with all its subfolders to the cloud, and in some cases to offer online collaborative document editing. One crossover product is SugarSync, which lets you sync folders wherever they exist on your drive.