What different between 'sheriff' and 'police officer'?

The most noticeable difference is jurisdiction. The sheriff's department has jurisdiction over the entire county, a police officer only has jurisdiction in the municipality that hires him (although there are some "county police" in some states such as Maryland).

The sheriff is elected, where the police chief is appointed. For that reason, he is the primary law enforcement officer for the county.

The sheriff also has additional duties such as running the county jail and serving civil process paperwork such as subpoenas and restraining orders.

Sheriff is head of a county peace officers, called Deputies. They usually are in control of the county jail, too,and serve papers.

Police are city.

One wears a stetson and a star and the other a cap and a shield.

...and both hate the other's taste in music.

Sheriffs work for the county. Police officers usually work for the city or town, sometimes for the state, the federal government, a railroad, etc.

They have different roles in different locations. It all depends on the law where you are talking about.

There is really no difference. A sheriff is a police officer for a whole county while someone with just the title of police officer is an officer for a city/town/municipality. A state trooper is a police officer for a whole state. And a security guard is a rent-a-cop with no police authority no matter how much he may pretend he does.

None as far as we're concerned.

Sheriff is the boss