A variety of professions involving some form of sex for hire. Traditionally these could include any of the following:

Prostitution   (escort, streetwalker, pimp)
exotic dance   (stripping, lapdance, peep show)
Dominatrix     (BDSM sessions for hire)
Pornography    (photographic, film, literature, webcam service)

Sex work is the primary sector of the economy where women routinely generate higher incomes than men (typically nearly double). This is not to say that in many cases men do not reap profits, however more than in the past, the businesses themselves are female owned.

In the US these businesses and the people who work in them are heavily stereotyped or regulated in both official and unofficial ways.

For instance, due to judicial interpretation of the state and federal laws it has been determined that in most states a municipality may not eliminate all venues for legal forms of sex work (e.g. strip clubs). As a result most towns and cities use zoning regulations to restrict these businesses to areas where they are least unacceptable.

Prostitution in particular is a profession and enterprise which is more or less tolerated (because it is virtually impossible to eliminate). One common social contract is that police expect prostitutes and prostitution to operate close to organized crime and expect these sex workers to cooperate in providing information when asked. This displays a sensible focus on relatively more serious crimes(sic).

Society's attitudes toward various forms of sexuality and sex work in particular creates some interesting contradictions:

Even "Soft core" pornography commands a high markup and sells well, this fact not withstanding, (normal) bookstores invariably put their sexuality titles in the back, where they quietly generate disproportionate levels of income.

Groups or businesses which come together to engage in practices which under the law look like sex work (e.g. BDSM clubs or private sex-play parties) can find themselves prosecuted as if they were a strip club or whorehouse.

While sexual images and references are obviously used to sell many commodities not directly related to sexuality, and sex work of all forms are multi-billion dollar markets. However we attach a social stigma to those who do sex work. This is part of what makes it a profitable business, and also creates suboptimal working conditions for many of the participants.

Sex work also carries real risks. STD transmission is a risk in the case of prostitutes and porn stars. Any sex worker takes on the increased risk to his or her safety, by the simple virtue of being different. Also in a world where many people inherently devalue sex workers, keeping one's self esteem can be a challenge.