In the US, the term can refer to a grand jury which issues an indictment or a petit jury which is present at a trial. When used alone the term usually refers to the latter.
In the United States, England and Scotland, defendants in most serious criminal cases have a right to trial by jury, although in practice most criminal actions in the US are resolved by plea bargain.
Juries are also used in many civil cases in the United States, and the United States Constitution explicitly protects the right to a jury in civil as well as criminal cases. In criminal cases, the right to a jury trial belongs to the defendant; if the defendant decides he or she is likely to do better without a jury, the prosecution cannot insist on one.
In English law, juries are only used in civil cases for criminal libel, or in some cases where the state is the defendant, such as some cases of wrongful imprisonment.
In most criminal justice systems which require juries, panels are initially selected at random from the adult populace. Individuals once selected are required by law to serve, often with exemptions for people whose job in some way precludes them (for instance, teachers, doctors, or people who themselves work in the criminal justice system), are caring for young children, or who have health problems. Most jurisdictions also limit jury service to citizens.
Jurors are then allocated to particular trials by a system of examination known as voir dire whereby both the prosecution (or plaintiff, in a civil case) and defense can object to a juror. Some jurisdictions give both the defense and prosecution a specific number of unconditional peremptory challenges, while most allow argument over whether a juror's particular background or beliefs make them unsuitable for service on the jury. Typically, there are twelve members of a jury panel, sometimes with a number of "alternates" who follow the trial but are not included in the deliberations unless a jury member is unable to continue for some reason.
In most civil law systems, a jury is not used, although lay assessors perform many of the functions of juries in some systems. In the US, juries are used in civil trials. In Japan, the post-war constitution guarantees the right to trial by jury, although this right is almost never used because it is believed that anyone who is desperate enough to want to have fellow citizens decide their fate is almost certainly guilty of the crime that they are charged with.
The jurors then hear the cases presented by both the defense and prosecution, and in some jurisdictions a summing-up from the judge. They then retire as a group to consider a verdict in secret, which they must reach unanimously in US criminal cases. On the rare occasions when no unanimous decision can be reached by the jury, a mistrial is declared, and the case must be retried with a newly constituted jury.
Some jurisdictions allow majority verdicts in criminal cases if a juror becomes unfit to continue, or if a judge permits it when a jury is deadlocked. At least 10 jurors must agree. Other than that, there are no restrictions on how a jury may proceed to reach such a verdict, and no set time limit. Occasionally juries deliberate for several days.
In the United States, some juries are also empowered to consider some aspects of a defendant's sentence, if the defendant has been convicted. This is notably the case in some death penalty cases. This is not the practice in most other legal systems based on the English tradition, in which judges retain sole responsibility for deciding sentences according to law.
The exception is the award of damages in English law libel cases, although a judge is now obliged to make a recommendation to the jury as to the appropriate amount.
The term "jury" is also used for groups of judges who decide on awards in various artistic areas, notably writing and film.
The period between 1830-1850 was when the modern adversarial jury trial took shape. See: D. J. A. Cairns, Advocacy and the Making of the Adversarial Criminal Trial, 1800-1865 (Oxford: Clarendon Press, 1998)
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