An expert witness is a person with specialized education, experience, or training who possesses a superior knowledge about a certain subject and can render a specialized opinion about that subject about which other persons having no training are incapable of rendering an accurate opinion or deducing a correct conclusion. An expert witness is hired to offer his or her opinion in a trial based his or her expertise. If an expert witness is qualified as an expert in a trial, he essentially assists the jury in understanding complicated and technical subjects not within the understanding of the jury (or the average lay person).
For example, in a murder trial the prosecution might call a handwriting expert witness to the stand to testify that a suicide note was not written in the victim's hand, but rather in the victim wife's hand. The expert witness's testimony about the handwriting would ultimately be offered to show that the victim had been murdered by his wife.