As late as the 1930's the US Supreme Court defined what a "Trial by Jury" meant, having the following 3 components: 1. A jury of 12 people 2. A judge with the power to instruct jurors on points of law and advise them on facts 3. A unanimous verdict. Trial by Jury is guaranteed in criminal prosecutions under Article III, section 2 of the US Constitution, which also has additional provisions pertaining to jury trials in Amendments 5, 6, and 7.
Historically, Trial by Jury developed under English Common Law as a means to protect an accused and check the power of the king or court, the jury alone being the means by which an accused's innocence or guilt was determined. In other words, Trial by Jury meant the common people would judge, as opposed to Trial by Government, where some governmental agency would determine guilt or innocence. Prior to this, "Trial by Ordeal," let God determine guilt or innocence, by having the accused forced into combat with another; God, in theory, would deliver an innocent from from an unjust punishment.
In Trial by Jury, the jury judges what are facts, what is law, and what was the moral intent of the accused. A little celebrated aspect of Trial by Jury includes the power of nullification, whereby a jury can "judge the justice of the law" and hold an unjust or oppressive law invalid. This means that if the accused is technically guilty of violating a law, but the jury determines that the violated law is unjust, the accused can be found innocent, and the offending law repealed. See more at the second link: