Translation from English to American, Please?

[Jim Flanagan]

No: HPFGUIDX 5874

Here's a quote of a quote from Ian Hart, playing Quirrell/Voldemort 
in the Movie. Can one of the bretheren or sisteren from across the 
pond translate the meaning of "posh" for us colonials?  I'm not sure 
what he's trying to say:

"The cast is brilliant," says Hart of the phenomenal line-up, which 
includes names as Richard Harris and Dame Maggie Smith, "Maggie's 
very nice, so's Alan Rickman, he's absolutely great as Snape.
I'm the only person who's not posh in the entire cast, and
I'm very happy with that." 

Thanks,
Jim Flanagan