Wikipedia's entry on Jury Nullification
"Jury nullification is a common term used to describe a jury's right to deliver a not-guilty verdict even where such a verdict clearly conflicts with the letter of the law."
"The use of the jury to act as a protection of last-resort was espoused by many of the Framers of the U.S. Constitution. As a result, the United States has a particularly strong legal tradition protecting the right of jury nullification. Though the right of a jury to nullify a verdict has been repeatedly affirmed by the U.S. Supreme Court, the 1895 decision of Sparf vs U.S. held that a trial judge has no responsibility to inform the jury of that right. This decision, often cited, has led to a common practice in which juries are instructed that they must find guilt or innocence according to the law."
What this means is that if you ever serve on a jury and you believe that the defendant is guilty but the law they broke is unjust you can declare them not-guilty anyway. They will tell you that you can't, but they're lying.