The party who starts a civil suit is called the Plaintiff. The party against whom a suit is brought against is called the Defendant. The lawsuit is started by delivery to the Defendant (called Service) of two documents: The Summons and the Complaint.
The Summons does just that: It calls the Defendant before the Court.
The Complaint also does what its name suggests: it lists the Plaintiff’s complaints against the Defendant. It will claim that the Defendant has committed some wrong against the Plaintiff, such as causing bodily injury or property damage or depriving the Plaintiff of something.
The Complaint will also ask the Court for relief (a remedy for the wrong), such as an award of damages (money) to repay the Plaintiff for his or her loss, or an Order to the Defendant to do something or stop doing something.
The Defendant responds to the Complaint with a document called an Answer, which responds to the Plaintiff’s claims, and explains why the Defendant believes the claim is untrue.
All of these documents, called Pleadings, are exchanged between the parties before the trial begins.
This describes a very simple civil case. It can be more complicated. There may be more than one Plaintiff or Defendant. The Defendant may also be asking for damages from the Plaintiff (a Counter-Claim), or from another Defendant (a Cross-Claim) or from someone else not originally involved but later added to the case (a third- party Defendant).
The Plaintiff or Defendant may not be individuals; they may instead be partnerships or corporations. A government (city, state, federal) may be a Plaintiff or Defendant. Whoever the parties are, the purpose of a civil trial is to decide disputes between them.