K-girl isn't in the FAQ abbreviations list.  CDL, GaG and others will chime in to correct me, but I'll start things off.
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K-girls are a subset of Korean Providers who are *generally*:
1. Korean from Korea, not Korean-American or Americanized Koreans.
2. Work for an agency and do not operate independently  
3. Usually work out of an apartment-like setting with 1, 2 or 3 K-girls sharing the premises at any given time.
4. **Provide an incomparable type and level of service that is appreciated by the K-girl lovers of this world.**  
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Exceptions. A more loose definition allows for:
Sometimes, a Japanese or Thai or Chinese girl will work for a K-girl agency and still be called a K-girl.
Sometimes, an Indy K Provider will provide a sufficiently similar (or superior!) experience to still be called a K-girl.
Someone who is too Americanized by birth, upbringing, or acculturation can provide a GREAT experience but not the same experience as an "authentic" K-girl. She might or might not be called a K-girl.  
Authentic K-girls are often imitated but never duplicated by wannabe K-girls.
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See also: http://www.theeroticreview.com/discussion-boards/k-girl-113/k-girl-7752?frmSearch=1#7752