Plaintiff vs. Defendant
The plaintiff initiates a civil lawsuit claiming harm; the defendant is the party sued, who must respond and mount a defense.
In a civil lawsuit, the plaintiff (also called petitioner in some proceedings) is the party who files the complaint, alleging that the defendant caused them harm and seeking a legal remedy. The defendant (or respondent) is the party being sued, who must file a formal response (Answer) denying or admitting the allegations. These roles define the parties' procedural obligations and the burden of proof throughout the case.
The plaintiff bears the burden of proof in civil cases — they must prove their claims by a "preponderance of the evidence" (more likely than not, i.e., >50%). This is a lower standard than the criminal "beyond a reasonable doubt" standard, which is why O.J. Simpson could be acquitted criminally but still found liable civilly for the same conduct.
These roles can become complicated: a defendant may file a counterclaim against the plaintiff (making the defendant also a plaintiff on the counterclaim), or third-party claims against additional parties. In class actions, the named plaintiff represents a class; the real parties in interest are the class members.
Real-World Example
The tenant (plaintiff) sued the landlord (defendant) for return of a $2,400 security deposit; the landlord filed a counterclaim (becoming also a plaintiff) for $800 in cleaning fees — the court awarded the tenant $1,600 net after offsetting the counterclaim.