The term "sheriff" generally refers to the head of a police department.
In the context of the law, the sheriff's department enforces the law and maintains security of the court. This can involve the secure imprisonment of criminal defendants, as well as the safe escorting of them to courtrooms from prison cells. Members of the sheriff's department also police courts simply to seep them safe from any crime that could potentially arise.
Sheriffs are typically given a specific territory such as a county. The sheriff's office will be responsible for enforcing the law in the county.
The sheriff's department also performs duties on direct instruction from the court. For example, a member of the sheriff's department may issue a restraining order to a particular individual. It is up to the sheriff to provide leadership for the department that they are running. For this reason, sheriffs are typically police officers who have a lot of experience and who have demonstrated leadership qualities.