Jury duty is every citizen's duty as well as being considered a right by some. When you pay taxes you are registered for jury duty in the county you live in. You can get called for jury duty for any trial at any time. Just because you get the letter does not mean you will be on the jury, you have to be selected at the courthouse out of several other candidates.
In important trials this is a very crucial part of the case, as having the right jury can assure victory or defeat. Some firms employ high priced jury selection companies that find out everything about a juror to help make a better informed decision.
Every jury appoints a foreman whom is like the team captain, he keeps order, counts votes, and speaks to the judge on behalf of the jury.
Sometimes the jury is kept under guard and not allowed to see anyone or speak to anyone due to leakage of media and other information that could affect their decision making process while serving jury duty. This process of isolation is called "sequestering" the jury.
The best part about jury duty (aside from the civic pride, yuk yuk) is the minimum wage pay that you collect on top of your normal pay from work that you missed, as your company is required by law to pay you while you are serving jury duty.
You can generally get out of jury duty if you have a good excuse, or if you don't want to serve just say something that you know will get you booted from the selection process such as expressing extreme right or left wing views or stating that you smoke crack.
"Mac, you ever been in love?" - "No, I've been a bartender all my life."