One of the best (in the sense of "most comical") regional dialects of American English, native to eastern Massachusetts. basically, any "ar"/"er" sound is replaced by "ah". In semi-technical terms, the accent is characterized by loss of the post-vocalic 'r' sound, e.g.:
car -> cah
mother -> moth·ah
garbage -> gah·bij
also: 'wicked' can be used as an intensifying adverb (like 'very') and a carbonated beverage is either called 'soda' or 'tonic', depending on location and the age of the speaker. Under no circumstances is it referred to as 'pop'.
A tricky accent to fake well; Robin Williams's attempt at Boston English in Good Will Hunting was laughably bad.