From LawGuru Wiki
A plaintiff, also known as a claimant or complainant, is the party who initiates a lawsuit (also known as an action) before a court. By doing so, the plaintiff seeks a legal remedy, and if successful, the court will issue judgment in favour of the plaintiff and make the appropriate court order (eg. an order for damages).
In some jurisdictions the commencement of a lawsuit is done by filing a summons, claim form and/or a complaint — these documents are known as pleadings — that set forth the alleged wrongs committed by the defendant or defendants with a demand for relief. In other jurisdictions the action is commenced by service of legal process by delivery of these documents on the defendant by a process server; they are only filed with the court subsequently with an affidavit from the process server that they had been given to the defendant(s) according to the rules of civil procedure.
Not all lawsuits are plenary actions. There are also simplified procedures, often called proceedings, in which the parties are termed petitioner instead of plaintiff and respondent, instead of defendant. There are also cases that do not technically involve two sides; such as petitions for specific statutory relief that requires judicial approval; in those cases there are no respondents, just a petitioner.
The party to whom the complaint is against is the defendant; or in the case of a petition, a respondent.
 United Kingdom
In England, Wales and Northern Ireland, the term claimant has superseded plaintiff as the official term for a person bringing a claim. This change was introduced by the Woolf reforms of the late 1990s, and is designed to make court terminology simpler for lay parties to follow. The reforms also abolished the term writ of summons in favour of claim form.