The different between wigs and hair pieces has to do with the portion of the head that is covered. In almost all cases a wig covers the entire scalp while a hair piece does not. A hair piece can cover part of the scalp or may be added in with the existing hair to make the hair look fuller or longer, or to create highlights within the hair. Just as wigs come in dozens of varieties and styles, so do hair pieces. While most wigs cover the entire scalp, there is quite a bit of variation in the way that hair pieces can be worn.
Many people consider wigs and hair pieces to be one in the same, but they create very different visual effects and are used for different purposes. Both appliances can be used to cover baldness, but only wigs can completely cover the scalp. If a person who is completely bald or has a shaved head wants to have the look of a full head of hair, she will have to wear a wig and not a hair piece. A person who only has a section of baldness might be able to cover this section with a hair piece.
Another similarity between the two is that they can both be worn for fun. Some people wear wigs just because they like to change their look from time to time. Some also wear wigs because they enjoy dressing in costume for special events. Hair pieces can be added to the hair to simply add volume or change the style of the hair. A person can wear a hair piece, for example, that makes it look as though she has bangs when she doesn't.
In most cases, the most expensive wigs and hair pieces are also the ones that are the least conspicuous. Very cheap wigs and hair pieces often look quite fake, whereas the most expensive ones can sometimes fool onlookers. The most expensive wigs and hair pieces are usually made out of human hair and a highly crafted. For those who plan to wear wigs and hair pieces on a regular basis, it sometimes makes sense to have the wigs trimmed or cut by a professional. This can help to make the wig or hair piece fit in better with one's natural hair or frame one's face in a more flattering manner.