The points a plaintiff must prove to win a given type of case are called the "elements" of that cause of action. (For the cause of action of negligence, for example, the elements are (existence of a) duty, breach (of that duty), causation (by that breach), and damages (incurred by the plaintiff).) If a complaint does not allege facts to support every element of the cause of action it describes, the court will dismiss the complaint for failure to state a claim, for which relief can be granted.
The respondent to a cause of action may plead denials or affirmative defenses. Some defenses, such as a particular court's lack of personal or subject matter jurisdiction may not be pleaded and may be raised at any time.