A witness in a criminal case is someone who testifies at a court hearing because they observed and/or have direct knowledge of a crime or significant event. They help lawyers verify versions of the events at trial and play an essential role in the administration of justice in a criminal case. A witness may have seen a crime, heard a shout during a crime or may be commenting on the character of someone.
Often times, a witness will be summoned to court by a subpoena. In most jurisdictions, a person must comply when subpoenaed by the court. In a criminal case if a subpoena is issued, the attorney of the defendant will most likely deliver the subpoena to those who will assist in proving the innocence of their client. On the other hand, the prosecuting attorney will seek to subpoena witnesses that will aide in proving guilt of the defendant.
At the court hearing, witnesses take an oath and are expected to tell the truth ("the truth, the whole truth and nothing but truth" as you may be familiar with in crime television shows) . If you are a witness in court, fail to tell the truth (lying) on the stand and are caught, you'll be charged with perjury.
There are several types of witnesses that may provide testimony in a court hearing:
An eyewitness brings observational testimony to the proceedings after having seen the alleged crime or a facet of it. It is sometimes unreliable (see "Reliability of witness accounts" below) however is presumed to be better than circumstantial evidence. When several people witness a crime, lawyers will often look for consistency among the recounting of events in order to determine what actually happened.
An expert witness is one that has superior knowledge to the average person when it comes to the topic they will testify about. These people are often doctors, forensic experts or psychologists.
This witness vouches under oath to the good reputation of another person often in the community where that person lives. The witness is there typically to say the defendant is a good person and possesses solid ethical qualities or morality. This kind of testimony is key when the defendant's honesty or morality is being question, which often happens in fraud cases.
There is much debate about the reliability of witness testimony. Part of the testimony can be meaningless and other parts can be downright wrong. Not in a way that the witness is perjuring themselves, but more simply their recounting of events is inaccurate. They may have a personal bias, the timing wrong, description wrong or some habit influenced them to perceive or believe they remembered something differently than it actually happened. The accuracy of witness testimony also diminishes with the person's age according to a study by Virginia University.
Additionally, there are a variety of studies that show the failures of witness memories when viewing police lineups due to being distracted by a weapon at the time of the offense or if there is a great delay in viewing the lineup from the time when the offense occurred. According to the Innocence Project (http://innocenceproject.org/understand/Eyewitness-Misidentification.php), "Eyewitness misidentification is the single greatest cause of wrongful convictions nationwide, playing a role in more than 75% of convictions overturned through DNA testing."