When a person has some dispute against another person he could move a civil suit before a Court of Law. Here the persons in dispute are the parties to the suit. Thus the parties are necessary element in a civil suit. In Civil Procedure generally there are two parties viz the plaintiff and the defendant. There may also be third parties who come incidentally to a suit.
A plaintiff is the one who moves a suit by filing a plaint. So he must have a claim against another in law. He must be one who is competent to sue. He must have the legal right and locus standi to file the suit. Order 1 Rule 1 of CPC explains who may be joined as plaintiffs.
A person who defends allegations and claims made against him is called the defendant. He must be competent to be sued. Order 1 Rule 3 of CPC explains who all may be joined as defendants.
All parties who are interested in the suit or its outcome can file applications in a Court of Law to add them as parties, either plaintiff or defendant. The defendant can also file application to join another person as defendant who shares the liability.