What is the difference between the Supreme Judicial Court of Massachusetts and the Supreme Judicial Court for Suffolk County?

The Supreme Judicial Court for Suffolk County, also known as the Single Justice Session is part of the Supreme Judicial Court of Massachusetts. Their powers are much like those of US Supreme Court Justices when they are sitting as Circuit Justices. In Massachusetts:

an associate justice essentially acts as a trial judge, or as an administrator of the court's supervisory power...

These sessions are held each week, and the associate justices rotate through them. The issues they hear include:

certain motions pertaining to cases on trial or on appeal, bail reviews, bar discipline proceedings, petitions for admission to the bar, and a variety of other statutory proceedings.

There's a more complete, but fairly accessible, description of the Single Court of Justice's "Practice and Procedure" here. For example, here are details about some of the Court's jurisdiction:

The single justice has jurisdiction over all interlocutory orders in criminal cases pending in the Superior Court, District Court, and in Juvenile Proceedings. In civil cases, with few exceptions, the single justice has jurisdiction over interlocutory orders of the District Court.