Short answers, pulled from the story.
A jury is a sworn body of people convened to hear evidence, make findings of fact, and render a verdict in a legal proceeding. Most trial juries, called petit juries, consist of up to 15 people who decide whether a defendant is guilty or civilly liable; the judge separately interprets the law and instructs the jury.
The jury system developed in England during the Middle Ages, with the Assize of Clarendon in 1166 establishing juries systematically across the country during the reign of Henry II. Earlier antecedents include jury courts in ancient Athens dating to 500 BCE and jury-like groups recorded in the Wantage Code issued by King Ethelred the Unready around 997.
A petit jury, or trial jury, hears evidence in a case and decides guilt or liability. A grand jury, by contrast, does not decide guilt; it examines evidence to determine whether enough exists for a criminal trial to proceed, and issues indictments. U.S. federal grand juries have between 16 and 23 members, compared with the up to 15 members of a trial jury.
Jury nullification occurs when a jury acquits a defendant regardless of the evidence, because the jury considers the law unjust or its application unfair. Notable examples include the 1670 trial of the Quaker William Penn, the 1985 acquittal of Clive Ponting under the Official Secrets Act 1911 in England, and the 1989 acquittal of Michael Randle and Pat Pottle for helping Soviet spy George Blake escape from Wormwood Scrubs Prison.
In the United States, prospective jurors are drawn from large databases such as driver's license and voter registration records and summoned by mail. They then go through voir dire, a questioning process conducted by the judge and lawyers, which allows challenges for cause and a limited number of peremptory challenges to remove jurors, before the final jury is impaneled.
According to a survey by Kalven and Zeisel (1966) covering approximately 8,000 criminal and civil trials, judges and juries agreed on the verdict 80 percent of the time. Suja A. Thomas has argued that, when a choice must be made, diversity, deliberation requirements, the need for consensus, and the absence of career incentives make the jury the least-biased decision-maker.