Both sell their bodies for something, money or drugs, or both in some cases.
One is just able to become famous for it and make a lot of money by showing her lady area for everyone to see for a profit. Porn is considered an art, that is protected by Free Speech, and I think that's a load of horse-crap. I don't think they should become famous and make that much money- but that's my opinion again, getting in the way; don't mind it.
Where as a 'Lady of the evening' that walks around street corners, will never be famous. They also have a far more dangerous 'job'. They run the risk of being kidnapped and being murdered because they try to keep it secret, since they'll be arrested because they aren't considered 'artists'.
Then again, you don't get diseases from wanking it in your bedroom to pornography. Not to say that porn stars, and streetwalkers alike, haven't gotten their fair share of diseases.
If you are talking about high-class broads who only do men for money, well, that wouldn't make them that much different. A camera is the only thing missing in that equation.