Just being a bit pedantic here, but 5mb says that 'booking out' and filing a flight plan are much the same thing. This is not the case. I'm not sure whether booking out is an idiosyncratic thing we Brits do but it means telling someone in authority at an aerodrome that we're going somewhere or that we've just come from somewhere. And, if we said we were coming but changed our minds, let the aerodrome know. (See Rule 20 for the full incomprehensible text)
Filing a flight plan is a little more formal than that and brings with it all sort of rules. The key thing though is that if you file a flight plan but down turn up, people will start looking for you (sometimes lots and lots of people). Regardless of the fact that you may have given your destination aerodrome a 'phone call to say you're coming to vist, the only way you are guaranteed search and rescue action it by filing a flight plan.
The waters got a little muddied a few years ago when we in the UK started using the term 'current flight plan' which simply means enough information to permit an air traffic service unit to manage a flight. If you're flying VFR this may well amount to little more than FROM, TO, POSITION and LEVEL .