The plaintiff begins a lawsuit by having a summons and complaint served on the defendant (usually by a sheriff). The defendant will then respond by filing an appearance and an answer.
The plaintiff's complaint must both:
recite the material facts of the case, so as to establish a cause of action or legal right to sue and
request a remedy, which usually will be monetary damages.
The defendant's answer will contain point-by-point responses to the statements made in the plaintiff's complaint. These responses will be comprised of admissions, denials or statements that the defendant doesn't know whether or not an allegation is true.
The defendant's answer also may contain:
an affirmative defense to the cause of action and/or
a counterclaim, wherein the defendant asserts a cause of action against, and seeks monetary damages from, the plaintiff .