Can anyone explain to me what's the difference between NYPD and NYC (city - I know) Police. I noticed also when I was on Boston that they seemd to have at least 3 police forces operating in the city, all with different uniforms (and that's not to mention campus police - are they a bona fide police BTW or just like armed security guards?).

How's a guy to know if he's being arrested by pukka police or being shaken down by a security organisation, especially as there's a multitude of different uniforms? I find this very confusing

In England & Wales there is only one police, with jurisdiction throughout the two lands (Scotland have their own), and then the Transport Police, who essentially police the railways. Sure, this force (or service they like to call it) is split into different county or regional units, but is essentially the same organisation, and has a recognisable uniform.

In France they seem to have the Police National, Police Municipal and the Gendarmes (more rural I think and sort of paramilitary), whereas Italy have something similar to the Gendarmes (Caribinieri) and Spain (Guardia Civil).