In personal injury actions the injured party (plaintiff) must prove negligence on the part of defendant, causation (the negligence caused the injury) and damages.
Negligence is usually defined as the lack of ordinary care and diligence required under the circumstances. The greater the potential danger, the greater is the care required. Fault comes into play when the jury deliberates at the end of trial. Basically, fault means negligence and there could be some fault attributed to both parties.
The jury on the verdict form is directed to assess a percentage of fault to any party who committed some negligence. The plaintiff will "win" the case so long as the plaintiff's fault, if any, is less than 50%. Should the plaintiff be found 20% at fault, the plaintiffs final damage award will be reduced by 20%.