The list below explains who they are and what they'll be doing.
Plaintiff (civil case) - In a civil case, the person who brought the case to court is called the plaintiff.
Defendant (civil case) - The person being sued in a civil case is called the defendant.
Defendant (criminal case) - A person who has been charged with a crime is the defendant in a criminal case.
Attorneys or counsel - Attorneys representing the plaintiff, defendant or the government in a criminal case are also referred to as counsel. Depending on who they represent and what court you are in, you may hear them called "counsel for the plaintiff," "plaintiff's attorney," "counsel for the defendant," or "defense attorney." An attorney representing the government in a criminal case is called the prosecuting or Commonwealth's attorney.
Court reporter - The court reporter keeps the official record by recording every word spoken during the trial.
Bailiff - The bailiff keeps order, maintains the security of the court and helps the judge and the jury as needed.
Clerk of court - The clerk of court, also called the clerk, maintains the court files and preserves the evidence presented during the trial. The clerk may also administer the oaths to jurors and witnesses.
Witnesses - Each side in a trial will probably have a number of witnesses who have information about the dispute. Very often, the judge will ask them to wait outside the courtroom until it is their turn to testify. This is done so they won't hear each others testimony and be influenced by it.