A lawsuit involves one person suing another person for something. In Supreme Court, the person who starts a lawsuit is called the plaintiff. The lawsuit is started by filing a notice of civil claim with the court registry. The notice of civil claim sets out the factual and legal basis for the claim, and what the plaintiff is seeking.
The person being sued is called the defendant. Once the notice of civil claim is served on the defendant, the defendant can file a response to civil claim that, similarly, sets out the factual and legal basis for opposing the claim.
If the defendant believes he has his own claim against the plaintiff, he can file a counterclaim, which is similar in form and content to a notice of civil claim. The plaintiff, once served with the counterclaim, can file a response to counter claim.
If the defendant believes that another person is to blame for the plaintiff’s losses, he can bring that person into the lawsuit by filing a third party notice. Once served with the third party notice, that person, now called the third party, can file a response to third party notice, setting out their position on the matter.
Collectively, these documents are called pleadings define all matters at issue in the lawsuit.