The U.S. justice and legal system requires courts to communicate with citizens via letters. Receiving a certified letter from a court official means that the court has communicated with you concerning whichever legal issue is relevant. Courts can use written missives to communicate with you for several reasons; usually they take the form of an order or summons to appear before a judge.
Every legally qualified citizen of the US must serve as a jury member as part of their civic duty, so you can expect to receive at least one jury summons in your lifetime. According to the United States Courts website, jurors are randomly selected from voter lists and you have to fill out a questionnaire to determine your eligibility. You are only exempt if you are under 18, cannot read or speak English, or are physically or mentally incapable of serving. In addition, members of the armed forces or rescue services are exempt. Exemption for medical reasons requires a letter from a physician.
A jury summons will typically include your court date, the time you are expected to appear, your juror badge number and parking information. The summons will include a telephone number you can call for assistance.
A court summons is a notice you must appear in court to answer a civil complaint. A summons must be delivered in person by a marshal, deputy marshal or appointed party. Alternatively, it can be delivered via certified mail. The summons will list the parties involved in the upcoming suit, the location of the court at which the hearing will be held and the contact information for the plaintiff's attorney--if one exists.
If you receive a summons, you must respond in writing or appear in person at the court specified. Usually you have a 30-day time limit in which to respond. Failure to respond to a civil summons allows the judge to issue a default judgment in favor of the plaintiff.
An injunction is a court order constraining the served party from doing a particular action. One example is a temporary restraining order. For the court to file an injunction against you, the plaintiff must show evidence that the action in question would cause irreparable harm. Injunctions are typically issued when monetary compensation alone would not remedy an action.
A subpoena differs from a court summons in that subpoenas are usually reserved for criminal trials. Subpoenas also provide a list of documents or items to bring or submit to the court for evidential purposes, such as tangible items, electronic documents or hard copies.