They decide if the defendant is guilty.
Twelve ordinary men and women are chosen at random. Their names come from the list of people who can vote in elections.
A jury must listen to all the evidence before they choose their verdict (say if the defendant is guilty or not).
This means that all 12 jury members agree with the decision.
If a jury cannot agree on a verdict then the judge might say that he will accept a majority decision. That means a verdict that 10 or 11 of the 12 jury members agree with. This normally happens after the Jury have spent a couple of days thinking about the verdict.
If the jury is split into two fairly even groups and people will not change their minds, then there can be no verdict. The jury is said to be hung.
The jury pick one member to be their spokesperson, they are called the foreman. The judge asks the foreman to tell the court the verdict.