A topic of non-stop argument and, apparent from other writeups here, considered to be pure evil with no positive aspects whatsoever.
Prostitution is the sale of sexual favours, usually (but not always) by a woman, usually (but not always) to a man.
This doesn't preclude the worker from living a happy, fun life. Typically in areas where prostitution is legal, those involved in the trade enjoy excellent pay, reasonable working conditions, excellent health, and occasionally a good lay.
Even in the United States, in the limited areas where prostitution is legal, little or no prostitution-related crime occurs, sexually transmitted diseases are non-existent (in Nevada, not a single case of HIV has ever been recorded), and customers usually leave with a big smile.
In legally licensed Nevada brothels, a prostitute lives on-premises, is provided a private room (one not used for servicing johns), off-hours entertainment (including internet access, satellite television, exercise equipment, etc.), and receives very good pay. She remains on-site to work for up to three weeks, then takes off however much time she wants or needs before returning to work. Whenever a worker returns to the brothel, she must be screened for HIV (and other STDs) and certified by a physician before returning to work. This is why most workers normally remain on-site for weeks, maximizing earnings and reducing the frequency of exams.
In such situations, prostitutes work as independent contractors, splitting basic fees with the house, keeping tips, and sometimes paying other costs out-of-pocket. These arrangements are commonly very lucrative for both contractor and brothel; a worker of average appearance may expect to gross $50,000 or more per year (this assumes two customers per day, paying $300 each, no tip, a three week working period of 15 days on and 6 days off, once per month, splitting fees down the middle with the house). Because the average prostitute works at least eight hours per day and could reasonably expect to have more than two customers per day, and because many look better than average (whatever that is), this estimate is extraordinarily low. Many prostitutes earn six-figure salaries. Those willing to participate in more lucrative activities, like group sex or servicing couples, base fees can skyrocket to $1,500 or higher, for a couple hours' work.
Anywhere prostitution is legal, safer sex is practiced either as required by law or by common sense. Even if a legal prostitute offers uncovered sex, it shouldn't be accepted unless you want to catch a disease or knock somebody up.
In environments where prostitution is illegal, more unfortunate situations can develop, as described at great length in other writeups here. These include indentured servitude, child prostitution, "crack whores", and so on.
Many people (both male and female) enter into prostitution because they feel they have no other options. Many others, however, enter into it because they believe they are attractive enough and skilled enough to earn ridiculous amounts of money under safe, controlled situations. Even in areas where prostitution is illegal, prostitutes of legal (i.e. age of consent) age enter into and remain in business by their own choice, and enjoy very pleasant levels of income. In such areas, the sex worker naturally keeps all the money instead of splitting it with a house or pimp.
Yes, some people are "trapped" in unpleasant situations that involve prostitution, but the rest are engaged in a profitable exchange of services for money. The world's oldest profession isn't the cause of all evil in the world.