In United States, the protection of rights and liberties in federal courts is achieved through the teamwork of judge and jury. The people don´t need any knowledge (ˈnɒlɪdʒ) of the legal system to be a juror. There are two types of juries in the federal trial courts: trial juries (also known as petit juries), and grand juries.
A civil petit jury is typically made up of 6 to 12 persons. In a civil case, the role of the jury is to listen to the evidence presented at a trial and to determine what the compensation should be. A criminal petit jury is usually made up of 12 members. Criminal juries decide whether (gueda) the defendant committed
the crime as charged
A grand jury, which normally consists of 16 to 23 members, has a more specialized function. The prosecutor in federal criminal cases, presents evidence to the grand jury for them to determine whether there is "probable cause" to believe that an individual has committed a crime and should be put on trial. Grand jury proceedings are not open for public observation.
Selection of Jurors
Potential jurors are chosen from lists of registered voters, or combined lists of voters and people with drivers licenses, in the judicial district.
In Spain, is regulated in Organic Law 5/1995 to try some criminal matters for example * Those crimes committed by civil servants while performing their public duties. * Crimes against the environment. * Offences against the person: murder * Embezzlement of public funds
These crimes will be tried in Provincial Audience, Regional high court and Sumpreme Court.
A list of candidates for jury service is issued every two years but only 9 members and 2 substitutes will be selected for each trial.
Finally, I think that the diferents betwen both sistems are: - number of juries and matters. In Spain is only criminal but in Unites State is civil too.