Some clothing is specifically underwear, while T-shirts and some shorts are suitable as underwear as well as outer clothing. Suitability as outer clothing is, apart from outdoor or indoor climate, largely a social and sometimes even a legal matter. One of the criteria for shorts not to be suitable as outer clothing may be that it has a fly that avoids exposure of the genitals just by an overlap of cloth, without buttons, etc.

In addition to keeping outer garments from soiling, undergarments are worn for a variety of reasons: warmth , comfort and hygiene being the most common. Undergarments are often used for modesty or erotic display; sometimes both of these motivations are simultaneously present.

Undergarments can also have religious significance, as in the special temple garment worn by followers of the LDS Church (Mormons). Some Jews use clothing that should neither touch the skin, nor be used as outer clothing: tallit katan or tallet ketannah or tzitzit. Every Jewish boy and man is supposed to wear Tzitzis, an undergarment, every day, and at a minimum while attending synagogue [1]. Some Hindus wear a Sacred Thread underneath their clothing that is supported on the left shoulder and wrapped around the body, falling underneath the right arm.

The two major types of men's underpants are boxer shorts (shorts-length and loose) and briefs (smaller and tighter), which are also referred to as Y-fronts in British English.

See lingerie for the types and various styles of women's undergarments.

From Wikipedia