Police oversee an incorporated city; sheriffs oversee a county. Police departments are established within the city limits of incorporated towns, cities, and other municipalities while sheriffs have jurisdiction over an entire county (mostly the unincorporated areas). For example, in Los Angeles County, the cities of Beverly Hills, Long Beach, Burbank, Pasadena, and others all have their own city police departments, while smaller communities like Palmdale, Malibu, and Santa Clarita are enforced by the Los Angeles County Sheriff's Department and do not have a police force of their own. In many cases, both the Sheriff's Department and a local police force will work together or split responsibilities; for example, in some jurisdictions a local police force will leave the Sheriff's Department in charge of running jails and serving civil papers (evictions, summons, etc.).

Police chiefs are appointed; sheriffs are elected. In most cases, Sheriffs are elected to office by a countywide general election (or special election in certain circumstances). Police chiefs, meanwhile, are typically appointed positions by high-ranking officials such as the mayor or city council for local police, or the governor for state police. There are certain exceptions, though, notably Rhode Island and Hawaii (both of whom appoint rather than elect a Sheriff), and Louisiana, which has both an appointed AND an elected Chief of Police in some municipalities.

Hierarchy. The Sheriff's Office and state and local police departments usually have slightly different hierarchies. Police departments are usually divided into - from lowest to highest rank - patrolmen/officers, sergeant, lieutenant, captain, and chief, while sheriff's departments are usually divided into - from lowest to highest rank - deputy sheriff and sheriff. If it's a particularly large department, a Sheriff's office may include additional ranks (Cook County, Illinois - which contains Chicago - for example, also has sergeants and lieutenants in the Sheriff's Department ranks. And a large police force might have even more intermediary and top ranks to accommodate a large number of personnel. The Chicago Police Department, for example, has commanders and deputy chiefs between the ranks of captain and chief... and the chief in turn answers to a first deputy superintendent and then a superintendent. The California Highway Patrol has a similar hierarchy going from officer to sergeant to lieutenant to captain to assistant chief to chief... and then through three ranks of commissioners.

Highway Patrol. The highway patrol is, in many cases, completely separate from both the Sheriff's Department and local police forces. Typically classified as "state police" in most states, they are entirely separate from both the sheriff and local police, having jurisdiction across the entire state with very specific responsibilities not covered by the Sheriff's Department or local police department. These include enforcing traffic laws on state highways and freeways, overseeing security of the state capitol complex and the governor, training law enforcement personnel for departments too small to have their own training program/facilities, and coordinating multi-jurisdictional task forces... for those pesky times when criminals are fleeing and don't stay neatly within the city or county limits.