In most counties in PA, the county jail is operated by the sheriff. Their primary duties are operating the county jail, providing security at the county court, and serving warrants issued by the county court. This can vary depending on the county's needs. For example Montgomery County PA has a Board of Prison Inspectors which runs the jail rather than the sheriff, but this type of board is unique to Mont. Co.

PA Sheriffs and their Deputies can arrest for felonies or breaches of the peace committed in their presence, or if they have a warrant from a judge. Under PA law they are not permitted to investigate crimes. This is indeed mostly unique to PA. In most states the Sheriff has full law enforcement powers, and in much of the south the Sheriff is the primary law enforcement agency responding to 911 calls in rural areas. Here that function is filled by PSP.

The only state with less powerful sheriffs that I know of is Delaware. DE Sheriffs can't make arrests or carry firearms.