What is a Complaint?
A Complaint, in law, is a formal written statement made at the beginning of a lawsuit in a civil court. The plaintiff "complains" of the wrong done to him by the defendant. The defendant's response to the complaint is called the answer.
The attorney for the plaintiff usually prepares the complaint, and sometimes the plaintiff is required to swear that it is true and sign it. The title of the complaint contains the names of the plaintiff and defendant, and the name and location of the court trying the case. A detailed statement of the facts that gave rise to the suit makes up the body of the complaint. Finally, a "wherefore" clause states the relief (usually damages) that the plaintiff demands. In some jurisdictions the complaint is called a declaration or petition.
A criminal complaint is the sworn statement of a person that a crime has been committed. It serves as a basis for criminal proceedings against a defendant.