They are noted by their notoriously thick accent, such can be heard in expressions such as "ey by! (hey boy / You dont say!)" or "whataya at? (What are you at / What are you doing atthis moment?)"
There is also a breed of jokes called Newfie jokes which poke fun at this breed of human, but ironically enough, Newfies are the creators of such jokes.
In insult, these people have been compared to "red necks"
God, he must a freakin Newfie, no one can ride a moose that fast!
That is to say, Newfoundlanders can call themselves and other Newfoundlanders "newfie" but if anyone who is not from Newfoundland uses it, it is considered offensive.
Newfie 2: Great, I love being a Newfie
Non-Newfoundlander: You Newfies crack me up!
Newfies: Watch your mouth.