http://en.wikipedia.org/wiki/United_States_magistrate_judge

It seems they usually deal with criminal cases, however also civil....

In civil proceedings, magistrate judges typically manage discovery and other pretrial matters. They are authorized to issue orders in pretrial matters as long as the order is not dispositive of the case as a whole (such as an order granting summary judgment). They may also be assigned to write reports and recommendations to the district judge as to dispositive matters. With the consent of the parties, they may adjudicate civil cases in the same manner as a district judge, including presiding over jury or non-jury trials.

Because Article III of the United States Constitution vests the judicial powers in courts to which the judges are appointed for life (and which are therefore called Article III tribunals), decisions of a magistrate judge are subject to review and either approval, modification or reversal by a district judge of that court, except in civil cases where the parties consent in advance to allow the magistrate judge to exercise the jurisdiction of the district judge. The magistrate judges therefore operate under the authority of Congress to appoint "inferior courts", set forth in Article I, making them Article I tribunals.

The Supreme Court most thoroughly delineated the permissible scope of Article I tribunals in Northern Pipeline Construction Co. v. Marathon Pipe Line Co.,[3] striking down the statute that created the original U.S. bankruptcy court. The Court noted in that opinion that the framers of the Constitution had developed a scheme of separation of powers which clearly required that the judicial branch be kept independent of the other two branches via the mechanism of lifetime appointments. However, the Court also found that Congress has the power under Article I to create adjunct tribunals, so long as the "essential attributes of judicial power" stay in Article III courts. This power derives from two sources. First, when Congress creates rights, it can require those asserting such rights to go through an Article I tribunal. Second, Congress can create non-Article III tribunals to help Article III courts deal with their workload, but only if the Article I tribunals are under the control of the Article III courts. The magistrate judges fall within this category of "adjunct" tribunals. All actions heard in an Article I tribunal are subject to de novo review in the supervising Article III court, which retains the exclusive power to make and enforce final judgments.

The Supreme Court later noted, in Commodity Futures Trading Commission v. Schor,[4] that parties to litigation could voluntarily waive their right to an Article III tribunal, and thereby submit themselves to a binding judgment from an Article I tribunal.