An accuser is a person who says that someone else did something wrong. The act of stating something wrong took place is considered an accusation. The person who says it is considered the accuser. This term is associated with criminal cases because it is understood that an accusation is the reporting of a criminal action. An accuser could be anyone in any type of case but is most often a victim of crime.
An accuser usually needs physical proof to back their claim; if they do not, it is considered circumstantial evidence. If they have physical proof, it is considered primary evidence. A court case will ensue when an accuser makes an accusation that is deemed by police to be probable by virtue of corroborating evidence.