What are the differences between 風俗 and 売春? jisho.org simply defines the former as "sex service" and the latter as "prostitution", but I'm not sure how different "sex service" is from "prostitution" when both of these forms of services seem to involve women giving physical sex service directly to customers. Does it have anything to do with the involvement of coitus (vaginal or anal penetration)? How are they defined by law? Which one is illegal?
There is a distinct difference in Japanese law. The Prevention of Prostitution Law of 1956 (売春防止法) defines 売春 as "engaging in sexual intercourse with all and sundry [不特定の人］in return for reward or the promise of reward". The courts have consistently interpreted "sexual intercourse" (性交) to refer to complete heterosexual intercourse involving penetration of the vagina by the penis, so no homosexual act or heterosexual act that does not involve such penetration is included. This is why it is pretty nearly impossible for the police to prosecute under this law - it's perfectly legal to offer, and even advertise, any services short of full (heterosexual) sex. And since such transactions usually take place behind closed doors, who's to know what goes on?
風俗, on the other hand, has a much wider application. The 風適法 (Law Regulating Entertainment Businesses) includes under the head of "entertainment businesses" (風俗営業) bathhouses, massage parlours, cafes, coffee-shops, cabarets, pachinko parlours, amusement arcades and so on. In other words, places that might be on the fringes of the sex industry or might otherwise tempt young people into undesirable habits. (The law is specifically intended, according to its preamble, to protect the moral well-being of the young.)
I'll leave comments on the use of these two words in general speech to native speakers, but my guess would be that 風俗 would be more casual and would refer to a wide range of relationships involving sex, while 売春 would refer only to sexual services provided for reward in cash or kind by professionals.