Dropbox has "selective sync", so individual PC's can choose to only sync a subset of folders from your account. Also, phones running Dropbox don't actually "sync" anything to the phone. You can choose to download individual files, or stream them straight from the Dropbox app, but it doesn't offer you any way to keep files on the phone up to date with whats in the "cloud".
I use dropsync on my phone. I have set up rules to sync screenshots to a different folder from photos, and periodically it will check a folder on dropbox and sync files from it to phone.
