Button-down shirts are rather unremarkable articles of clothing. They can be either long-sleeved or short-sleeved, loose fitting or form fitting, male or female or unisex. They are standard issue yuppie wear, especially with khakis, and can often be spotted on the backs of patrons at Starbucks. Crisp white, white-striped, or powder blue button-down shirts are regularly seen peeking out from underneath the lapels of a double-breasted power suit, held securely around the wearer's neck by a necktie of some kind.
Flannel button-down shirts, on the other hand, are often worn by teenage boys, grunge rockers, and Canadians (at least those in British Columbia, anyway). In these cases, they are often seen as symbols of rebellion or a deep-seated need to stay warm.
Button-down shirts can also be worn unbuttoned over a t-shirt, tank top, or other shirt-like clothing item. In this respect, the basic button-down shirt can serve as a light jacket or cover-up.
Interestingly enough, men born in India tend to wear their button-down shirts untucked at all times (except when wearing business suits), owing perhaps to the loose-fitting style of their traditional garb. Men of Indian descent who were not born in India generally tuck their shirts in, unless they are wearing them over another shirt. Someone ought to conduct a study on this phenomenon.