Newbie - FAQ
Those terms were introduced on a web site that offers free provider web sites. As the free sites have become more popular recently, some of the terms have gotten more exposure and use.
Here is a partial list of those definitions (the ones that do not appear in the TER 'Definitions, Acronyms, Abbreviations, and Terms' post - see link below):
A-Level (5 star escort)
COB (Come On Body)
COF (Come On Face)
Completion (Oral to completion)
Duo (Sex with two escorts; Threesome with the client)
Extraball (Have sex many times)
LT (Long Time; Usually overnight)
O-Level (Oral sex)
OWO (Oral without a condom)
I have no idea how they define a 'five-star escort', or conversely what it means to be either a '3-star' or D-Level.... I think they just made some of that stuff up. 
-- Modified on 7/25/2006 2:30:57 AM
I can't comment on the site which originated the terms, but I can say that the "O-Level" and "A-Level" business looks to be derived from the British school system. At least, that's how they refer to the different levels of schooling attained, what we might call a "GED" vs. "Master's Degree" in French or whatever.
Kinda makes sense 