I should probably let slice have this one but FSS provides weather briefings, notams, TFRs and files IFR/VFR flightplans (among other things). When you file an IFR flight plan with FSS, they will punch it into the system and ATC will receive it. When you call clearance and say "N123 would like to pick up an IFR clearance to LGB" they will have your plan on file and read you your clearance. With a VFR plan, you have to call an FSS radio and activate the flight plan. Sometimes the FSS will transmit through a VOR but out here in SoCal, they usually have a direct radio. Also, I have activated VFR fp's over the phone and sometimes SoCal approach will do it for you if they're not overloaded. But like slice said, VFR will not guarantee traffic separation on ATCs behalf. VFR flight plan also doesn't guarantee that you'll be able to talk to anyone either. Most of the time they're too busy. But a VFR FP is a good safety net; in case you go down somewhere, search and rescue will try to find you on your filed VFR route if you don't call FSS and close the FP after you land. My PPL instructor had that happen to student on a CC solo.