The 'F' suffix stands for Freight. It can vary from day to day. e.g.today's and tomorrow's BA193 to Dallas are passenger (and freight) flights but the following day (23rd) it's BA193F denoting freight only. Note the ATC callsigns are unchanged (BAW31F). Just to add to the confusion, the 'F' in the callsign BAW31F doesn't signify a freighter. It's just an alphanumeric callsign to avoid confusion with another flight with a similar callsign.
The main differences between the two from an operational perspective are cabin crew are needed if passengers are carried but a freighter is flight crew only and a passenger flight needs access to a passenger terminal building but a freighter can park on a freight stand.