In a trial with a judge and a jury, each has very different parts to play.
The jury listens to the evidence and decides who or what to believe. It decides what the facts of the case are. Jurors are the only ones who can decide whether the accused is guilty or not guilty.
The judge ensures that the proper legal procedures are followed. Because the jury does not decide these questions, many discussions and decisions related to the law take place when the jury is not in the courtroom.
When the jury reaches a verdict (decision on the guilt or otherwise of the defendant), its role comes to an end.
If the jury finds the defendant guilty, it is then the role of the judge to decide the appropriate sentence.