The first papers filed in a lawsuit are called the pleadings. These include the plaintiff’s complaint and then (usually after thirty or more days) the answer or response from the defendant. The answer may be coupled with a counterclaim against the plaintiff. (In effect, the defendant becomes the plaintiff for the claims she has against the original plaintiff.) The plaintiff may reply to any counterclaim by the defendant.
State and federal rules of civil procedure require that the complaint must state the nature of the plaintiff’s claim, the jurisdiction of the court, and the nature of the relief that is being asked for (usually an award of money, but sometimes an injunction, or a declaration of legal rights). In an answer, the defendant will often deny all the allegations of the complaint or will admit to certain of its allegations and deny others.
A complaint and subsequent pleadings are usually quite general and give little detail. Cases can be decided on the pleadings alone in the following situations: (1) If the defendant fails to answer the complaint, the court can enter a default judgment, awarding the plaintiff what he seeks. (2) The defendant can move to dismiss the complaint on the grounds that the plaintiff failed to “state a claim on which relief can be granted,” or on the basis that there is no subject matter jurisdiction for the court chosen by the plaintiff, or on the basis that there is no personal jurisdiction over the defendant. The defendant is saying, in effect, that even if all the plaintiff’s allegations are true, they do not amount to a legal claim that can be heard by the court. For example, a claim that the defendant induced a woman to stop dating the plaintiff (a so-called alienation of affections cause of action) is no longer actionable in US state courts, and any court will dismiss the complaint without any further proceedings. (This type of dismissal is occasionally still called a demurrer.)
A third kind of dismissal can take place on a motion for summary judgment. If there is no triable question of fact or law, there is no reason to have a trial. For example, the plaintiff sues on a promissory note and, at deposition (an oral examination under oath), the defendant admits having made no payment on the note and offers no excuse that would be recognizable as a reason not to pay. There is no reason to have a trial, and the court should grant summary judgment.