Hi @logr; welcome to the Dropbox Community and happy Monday!
On one hand, sync, being one of our desktop app's core features, keeps your files up-to-date across your computers, phones, and tablets. If you edit a document within your Dropbox Folder on your computer, it syncs with Dropbox and you can access the new version from your phone and our website.
Computer backup , on the other hand, lets you back up certain key folders on your computer, such as “Desktop”, “Documents”, and “Downloads”, to Dropbox. Any changes you make to those folders on your computer or in Dropbox will continuously sync to both.
None of those backed up folders will take up any additional space on your computer while anything you delete from them or dropbox.com will be deleted from both your computer and Dropbox.
I hope this clarifies matters a bit further and I'm always around for any follow up questions.