Air Traffic Assistants work in centers, doing flight data. I don't know everything that they do. Amend incorrect IFR flight plans? along with some other stuff.
Flight Service, depending on where you're at, mostly deals with VFR aircraft. filing/activating/closing VFR FPs, initiating Search and Rescue when pilots go overdue, giving preflight weather briefings, inflight weather briefings over the frequency, relaying information from Center to IFR pilots that are too low for Centers radios, and airport Advisories for uncontrolled airports. From my experience in southeast Alaska, FSS here is quite a bit more involved with pilots(IFR and VRF) and center controllers, due to terrain and radio coverage.