Private First Class. At one time there were specialists all the way up to SP8, the same pay grade as a First Sergeant. A junior NCO with 3-8 years of service. Sergeants are usually responsible for several privates and specialists. A junior NCO with 6-12 years of service.
Staff sergeants are usually responsible for a squad or one armored vehicle.
Sergeant First Class. A senior NCO with 10 or more years of service. A sergeant first class is the top enlisted person in a platoon. Master Sergeant or First Sergeant. Both are senior NCOs.