A poutine is a french fry, that's been topped with cheese curds, and then a chicken gravy has been poured over top. It melts the cheese and gets all gooey and wonderful. People here in Quebec often refer to it as a heart attack in a bowl.
http://www.scores.ca/DATA/TEXTEDOC/ST_poutine_classique.png
The closest thing to this in USA I would say was the Taco Bell cheese fries. But they use chili or salsa instead of gravy, and their cheese is more like Velveeta. It does not have that mozzarella like texture that the cheese curds have.
http://static3.businessinsider.com/image/57fd08ec8109eeb6428b4a27-1200/taco%20bell%20fries.jpg
By the way, there are many restaurants in Montreal making variations of the poutine you can try. Here's the menu from the famous Banquise restaurant in Montreal.
http://labanquise.com/en/poutine-menu.php
If you are looking for poutines in USA, best places to try are upstate New York and Vermont, or down in Florida where all the Canadian "snowbirds" hang out during the cold winter months.