Shorts are a garment worn by both men and women over their pelvic area, circling the waist and splitting to cover the upper part of the legs, sometimes extending down to the knees but not covering the entire length of the leg. They are called "shorts" because they are a shortened version of trousers, which cover the entire leg. Shorts are typically worn in warm weather or in an environment where comfort and air flow are more important than the protection of the legs.
There are a variety of shorts, ranging from knee-length short trousers that can in some situations be worn as formal clothes to beachwear and athletic shorts. Some types of shorts are typically worn by women, such as culottes, which are a divided skirt resembling a pair of loose-cut shorts.
In British English the term "short trousers" has been used, but only for shorts that are a short version of real trousers (pants), e.g. tailored shorts, often lined, as typically worn as part of school uniform for boys up to their early teens, and by servicemen and policemen in tropical climates. The American term "short pants" is probably the nearest equivalent in the US, there they might now be called "dress shorts", a term that has not gained much currency in Britain. A somewhat similar garment worn by men in Australia is called "stubbies". "Shorts" is used unqualified in British English to refer to sports shorts, athletic shorts, or casual shorts: the last is nowadays commonplace in warm weather in the UK.
The term "boxer shorts" was an American coinage for a particular kind of men's underwear, and is now common in Britain. However, boxer shorts are often referred to as "boxers" in the USA. The word "pants" refers to outerwear (trousers) in American usage (except when modified to "underpants") but when used unqualified in British English generally means underwear.
In much of Europe and the Americas during the 19th and early 20th centuries, shorts were worn as outerwear only by young boys until they reached a certain height or maturity. When boys got older, typically around puberty, they would receive their first pair of long trousers. This produced the perception that shorts were only for young boys. Because of this, men would not wear shorts to avoid looking immature. Women tended not to wear shorts in most cultures, due to social mores: they were expected to wear dresses, or skirts and blouses.