A special prosecutor is a lawyer from outside the government appointed by the attorney general or Congress to investigate a federal official for misconduct while in office.
The reasoning is that the Department of Justice may have political connections to those it might be asked to investigate. Inherently, this creates a conflict of interest. The solution is to have someone from outside the department lead the investigation.
Specialprosecutors have investigated such notorious cases as the 1920's Teapot Dome bribery and corruption affair and the tax scandals of the early 1950's.
Actually appointing the specialprosecutor was a new three-judge panel, based at the US Court of Appeals in the District of Columbia.
All told, the specialprosecutor law was invoked eleven times from 1978 to 1982, and resulted in three appointments of a specialprosecutor.