from what I gather it refers to a particular type of Korean gal who offers excellent though perhaps less than personal service for a very reasonable price, mostly centered around LA, but seems to be spreading out from there.  They are quite high volume.

 
Getting into the fraternity that can visit these highly regulated gals is somewhat problematic.

as Fisher suggests. There have been more than a few discussions on that topic over the years.

 
However, give the search functions here are a bit limited I'll offer my summary. K-girls are Korean providers that were raised in the Korean culture (or generally rules out Korean-Americans born in the USA) as that culture imparts certain characteristics to the male-female relationship, roles and responsibilities. Some will say they cannot be independents but need to work in one of the Korean Agencies. Some will be permanent residence in the USA while others will be the 90 day wonders -- visiting on the visa on entry status Koreans have to come to the USA for up to 90 days without needing to apply for some visa in advance. They are generally not the most expensive but offer very good service and seem to take personal pride in doing the job well. They are not submissive but I think much better describes as being very good hostesses. They often have a common "script" as some call it so it becomes a recognizable experiences.