What is the main role of the jury?

The purpose of a jury is a render a verdict in a criminal trial. The underlying idea is that the decision should be rendered by common citizens who are impartial to both the individuals involved in the case as well as being impartial to the political system. As a result, they are viewed to add a degree of fairness to the proceedings.

The first answer is not entirely correct. Juries are used in both criminal and civil matters. Their function is that of the finder of fact. That is, a trial involves the presentation of evidence by both the sides of the matter. Evidence can be documents, testimony or other types. A jury sifts through usually conflicting facts.. In a civil case, it determines the prevailing party in a trial based upon a "preponderance of the evidence" (meaning, essentially, that one party has "tipped the scales of justice" a slight amount in its favor). In a criminal case, the burden of proof is greater, in that the prosecuting authority must present facts tending to prove guilt "beyond a reasonable doubt" (which is a greater burden of proof than in a civil case).

Juries in civil cases seeking money damages also determine the amount of money awarded (if the Plaintiff prevails). In a criminal case, the statute(s) that the Defendant is alleged to have violated generally prescribe the penalty, so a jury is not always involved at that stage. However, if the matter carries the potential of the death penalty and the Defendant is found guilty of the crime, a new jury may be convened to determined is the death penalty or a lesser penalty will be imposed.