I just read this this question about MathOverflow and Mathematics. After reading the answers and comments, I get and idea that every site that contains "Overflow" is meant for experts. MathOverflow is for expert mathematicians, Stack Overflow is for expert programmers, and Physics Overflow is for expert physicists. Will someday we will have Health Overflow, Language Overflow (should we change Linguistics to this name?), Hack Overflow (on Area 51), etc?
PS: I know that Physics Overflow doesn't belong to SE
As in ChrisF's answer, Server Fault is for expert withou the tag "Overflow". But that's is a legacy site. Maybe starting from now, should we name site for experts like that so that new comers can quickly indicate which site they suit ask.