Police handle law enforcement in a city or town. It depends on the size of the town whether or not there is a police force. The Police is run by the Chief of Police, then detectives (who investigate crime), and police officers (who handle law enforcement). The Chief of Police is hired by the city and not an elected position.
Sheriffs handle the law enforcement of the county. Each Sheriff has Deputies who work for him/her (equivilant the police officers of the county). Sheriff is a elected position. My mom lives approx. 5 miles outside of her local city and when her house was broken into she had to call the Sheriff. And it took a while for the Deputies to arrive, since there is only 2-4 of them in her county. And since a county can be quite big (it can take over an hour to travel from one end to the other, taking into account the country roads, etc.), so unless the Deputies are nearby, the county folks are mostly on their own. Deputies handle both investigations and law enforcement.
Then there is the Highway Patrol. They are hired by the state and considered our state's law enforcement and mostly deal with law enforcement along the states Highways and State Roads (they are able to do law enforcement between counties). So, if a chase crosses county lines, or a murder happens between counties, or another crime... they are the ones that get called in.
I live in a town/city of approx. 35K-40K, surrounded by farming community (very similar to the Smallville on Smallville). We have a Police Department, plus we also have the Sheriff and have Highway Patrol (and since we're not close to an actual "Interstate Highway", I figure they must handle law enforcement for the State Roads as well). And then we have the prison and their guards. But that's a whole other mess of beans and probably why our small town is increasing in gang activity.
Hope this helps.
