Normally, in every court there is a plaintiff or prosecution, and a defendant. The plaintiff is the one who files the complaint (or in the case of a criminal trial, the charges), while the defendant is the one against whom the action is taken. In the case of a criminal trial, the defendant may also be known as the accused.
Normally, when one party files a complaint against another, the latter is served by an officer of the court. This means that the documents outlining the nature and details of the complaint must be personally delivered to the defendant.
Once the defendant has been served, s/he, or his/her representative, must respond within a specific time period, or the judge will automatically decide in favor of the plaintiff.
If the plaintiff brings a complaint against multiple parties, these are known as co-defendants.
A defendant may be an individual, a group of persons, or a business entity such as a corporation. A plaintiff may also file suit against an institution such as a school district or religious organization (which happens on a frighteningly regular basis - teachers today are often disciplined and even dismissed on nothing more than an accusation), governmental agency, and even the government itself.
Whereas the defendant in a criminal case is normally taken into police custody and must appear in court in person to answer the charges, the defendant in a civil case needn't show up in court at all. Of course, failing to do so may result in a default judgment in favor of the plaintiff. However, defendants may also have an attorney represent them in litigation without their actual presence in the courtroom.