A special prosecutor is an attorney who is hired by the government to act as a prosecutor in special circumstances when a normal government prosecutor is not preferred. Circumstances that commonly call for special prosecutors are situations where a public law official, such as a judge or a district attorney, is facing a criminal trial.
The reason why the government often prefers special prosecutors to government prosecutors in situations where a judge or DA is the defendant is because it is often feared that a government prosecutor may have a conflict of interest with the defendant. In other words, the government uses an outside lawyer to prevent risking using a lawyer with direct personal and work-related ties to the defendant.
Special prosecutors must have adequate knowledge of criminal law to be qualified to take on such cases.