Specifics vary from state to state I believe. The basics: You are selected either by being a registered voter or having some other form of government identification (Driver's License). It is normally the former. Requirements are: United States citizenship, residence in district court is in, age of 18, conversant in English, physically and mentally able to carry out the functions of a juror, no service on jury in last year, and no felony convictions. Exceptions can be made to excuse people whose service would negatively impact others (ER doctors, people caring for sick relatives, etc.) People with prior knowledge or bias against defense or prosecution are also excused, except in cases where prior knowledge is unavoidable (famous or sensational crimes and criminals). Inconvenience is not an excuse. If chosen, both defense and prosecution can choose to dismiss you on various grounds. Some not chosen are kept as alternates. A total of 12 is necessary. Pay is a pittance, normally less than $10 a day. On the plus side it is illegal for an employer to fire you for missing work in jury service. Meals and accommodations, if necessary, are provided. Duties are to respond to summons, and if chosen to serve, be prompt and stay the whole time. You are to elect a foreman, who will lead discussion and give the verdict to the judge. All evidence must be weighed as impartially as possible, and deliberations made with care and intelligence.

I just want to add that I was called for jury duty not too long ago. Though I wasn't actually selected, I was still paid 40 dollars plus a certain pay per mile it took for me to get there and back. The total I was paid was 57 dollars. Annoying as all heck that I had to sit there for 4 hours literally being bored out of my mind, but...well...it's money.