An expert witness is a specialized kind of witness. Whereas most witnesses are meant to provide firsthand testimony regarding a crime, expert witnesses are called to testify in a court instead to provide some form of important information based off their own unique and specialized knowledge. Expert witnesses must always be qualified in some way to provide this information, though such qualification can come from a variety of different sources, including training, education, and professional experience.
Expert witnesses might also come from any of a number of different backgrounds and areas, as the testimony provided by an expert witness will only matter in context of the overall argument being made by one side of the trial. In some trials, for instance, an expert witness might be called to testify to provide information with regards to a particular piece of equipment which might be involved in the trial, whereas in other trials, an expert witness might be called to testify to provide an objective third party understanding of the extent of an injury. These are just examples of the many possible functions for which one side of the trial might call expert witnesses.
Expert witnesses are sometimes criticized by those analysts of court procedures, as expert witnesses, despite theoretically having to conform to a high level of standards and honesty with regard to their expert testimony, will often take advantage of their status as expert witnesses to provide explanations and accountings which are not actually accurate. If both sides in a trial call an expert witness, as well, it will be left to the jury to decide which expert witness to believe, which is considered to be a non-ideal situation.