I am sure there is already a thread or FAQ about this somewhere, but I wonder what it means when an ad says GFE. Alternately, I also wonder what it means when they specify NO GFE. What's the difference?

Now, that means different things to different people. To me, GFE means lots of kissing and she'll probably give you a BBBJ, but this isn't always the case. It's more of an attitude where she makes you believe she's into you.  

However when you see "nonGFE" in an ad, you can generally conclude that provider does not kiss and does not do BBBJ.  
If a provider advertises she IS GFE, you can't tell what that really means. It does not necessarily mean she does do DFK and BBBJ. Some people consider GFE included certain services provided. Others say it's about attitude and personality. The best way to determine a provider's service and attitude are reviews and backchannel info, not by an acronym in an ad.

... FS and non-FS.  But as others said, there's no one definition of GFE.  Or non-GFE.  You need to do your research to find out what THIS provider means.

it means whatever the provider decides it means for her.  As my esteemed colleagues above have said, there are varying degrees of what is involved.  I do agree that if she is non-GFE, there is probably no DFK, or maybe kissing at all.

BBBJ and DFK.

and to others; GFE is an attitude.. not a checklist.

So in that case I would ask you, as a provider, what is the difference in attitude?