Worn under everything else, underwear includes panties, breifs, bras and so on. Underwear is often seen as alluring or erotic, particularly women's lingerie.
Underwear is clothing worn over the crotch, buttocks, and women's breasts. As the name implies, it is the clothing worn under regular clothing and typically isn't visible from the outside. In most cultures a person's underwear is considered very private.
Women typically wear panties and brassieres (commonly known as "bras") which come in a wide variety of styles, and are often designed to be sexually appealing. Women also sometimes wear additional undergarments like garterbelts, stockings or tights.
In anime and manga, the color, style, or type of underwear often carry particular connotations.
Men typically wear breifs or boxer shorts and occasionally vests or undershirts, which come in a range of types and styles and are typically designed for functionality and comfort rather than sexual appeal.
Often in anime and manga, a character caught in their underwear becomes incredibly embarrassed, sometimes even violent. Underwear can also be the thing of perverse obsession for some characters. Franchises that tend to have major ecchi and/or fan service themes are virtually guaranteed to feature women's underwear, panties especially often via the infamous panty shot.