In addition to Jura's correct explanation, let me add that not all American trials involve a jury. In many cases, the person on trial decides if s/he wants a jury or just a judge. This is especially true in civil cases (cases involving money, not crimes).

In a trial with a judge and a jury, each have very different parts to play. The jury listens to the evidence and decides who or what to believe. They decide what the facts of the case are. They are the only ones who can decide whether the accused is guilty or not guilty. The judge sees that the proper procedures are followed and she or he makes decisions about all questions of what the law is in relation to the particular case. Because the jury do not decide these questions, many of the discussions and decisions related to the law are made when the jury is not in the courtroom. When the jury reaches a verdict (that is, decides whether the defendant is guilty or not), its role comes to an end. If the jury finds the defendant guilty, it is then the role of the judge to sentence the defendant.