A Bratwurst is a fried sausage. Usually eaten with mustard and bread, it is a popular snack in German-speaking countries which is sold at various fast food outlets and often consumed standing up.
A Bratwurst is also part of a traditional dish served at restaurants. It includes a fried sausage, Sauerkraut and potatoes.
In the United States, the term bratwurst is used to describe such a sausage, regardless of how it is cooked. This is usually a misnomer, as these "bratwurst" are generally grilled, so that grillwurst would be a more accurate term. Like German bratwurst, the American variety is usually eaten with bread (a hot dog bun, for example) and topped with mustard and many of the other condiments often eaten with hot dogs.