Bratwurst is a dish of German origin. It is a type of sausage. It is composed of pork or beef, and sometimes veal.

Where the word comes from
Bratwurst is a sausage in German cuisines, from Old High German brätwurst, from brät which is a derivative of the Old High German word "brato". "Brato" originally meant hacked meat, intestines. The other part of the word is wurst which means sausage.

Where the sausage comes from

Though originally Celtic in origin, the German "Bratwurst" comes from Franconia, specifically Weißenburg-Treuchtlingen.

Major Types of German Bratwurst 
There are many different kinds of these sausages. All are special in their own way:
 Coburger Bratwurst: A Bratwurst originating in the city of Coburg in Bavaria. It is made from a minimum of 15% veal or beef, and its seasonings include only salt, pepper, nutmeg, and lemon zest. It is coarse in texture and measures about 10 inches in length. Traditionally it is grilled over pinecones and served in a bread roll (Brötchen).
 Fränkische Bratwurst: A relatively long (4-8 inches), thick, coarse sausage, originating from the Franconia (Franken) region in Bavaria. It dates back to 1573. The Fränkische Bratwurst is traditionally served with sauerkraut or potato salad, but with no mustard. 
 Kulmbacher Bratwurst: The Kulmbacher Bratwurst, from the city of Kulmbach in Bavaria, is made mainly from finely ground veal. They are long and thin. 
 Nürnberger Rostbratwurst: A small, thin bratwurst from the city of Nürnberg. It is no longer than 3-4 inches and weighs no more than 1 oz. They are traditionally served is sets of 6 or 12 (depending on your appetite) with horseradish and sauerkraut or potato salad.
 Nordhessische Bratwurst: The Nordhessische Bratwurst (from Northern Hessen) is similar to the Thüringer Rostbratwurst in taste. It is made from coarsely ground pork and is heavily seasoned. It measures around 8 inches in length. Traditionally, it is grilled over a wood fire and served on a cut-open roll (Brötchen) with mustard.
 Rote Wurst: The Rote Wurst is a favorite Bratwurst of the Swabian region. It is similar to the Bockwurst, and is made from finely ground pork and bacon. Its taste is spicy. To prevent splitting during grilling or pan frying, an X is cut into the ends of the sausage. The ends open during cooking, but the rest of the sausage remains in tact, giving it its traditional shape.
 Thüringer Rostbratwurst: The Thüringer Rostbratwurst is a spicy sausage from Thüringen. It is long (6-8 inches) and thin in shape. Traditionally, it is grilled over a wood fire and eaten with mustard and bread.
 Würzburger Bratwurst: The Würzburger Bratwurst, also known as the Winzerbratwurst, comes from the city of Würzburg. It's size is similar to the Thüringer Rostbratwurst, but its ingredients include white Franken wine.
A "Bratwurst meal" often is eaten with sauerkraut, mashed potatoes and roasted onions. Sometimes, it is also served with a potato sauce, or a potato salad.

How the sausage is prepared 
Today, most kinds of Bratwurst are prepared by roasting them. Depending on the kind of Bratwurst, it may have been cooked beforehand. Sausages that use raw meat, and that are not roasted or cooked before they are eaten are usually not called Bratwurst.

How people eat the sausage
The sausage is usually eaten with a hot or sweet German mustard or sliced and eaten as Currywurst. Sometimes, some hot French mustard is served as well. Bratwurst is almost always served with a hard German roll and usually accompanied by a beer. It is a popular snack in German-speaking countries, where it is sold at various fast food outlets and is often consumed while standing.

Bratwurst in the US
In the United States, bratwurst are usually eaten with bread (a hot dog bun or a hardroll, for example) and topped with mustard and/or many of the other condiments often eaten with hot dogs, including onions (grilled and/or raw), relish, sauerkraut, etc. The bratwurst is occasionally eaten with a pair of brat links nestled in a buttered hardroll with these same toppings, which is called a 'double brat'.

Within the US, bratwurst is strongly identified with Milwaukee, Wisconsin, and other areas in a band stretching from the north side of Chicago up through Minnesota and scattered pockets elsewhere in the Midwest. 

They are especially popular in areas of the US where German-Americans settled in large numbers, like Sheboygan, Wisconsin, which is informally known as the Bratwurst Capital of the World because of the city and county's very strong German roots and connections to bratwurst. Johnsonville Foods, the nation's largest bratwurst maker, is based in the nearby unincorporated village of Johnsonville. The city also celebrates Sheboygan Bratwurst Days, a community festival held on the first Thursday-Saturday of August each year that celebrates the bratwurst.  Other traditional Wisconsin brat manufacturers include Klement's Sausage Company and Usinger's, both of which are based in Milwaukee.

The city of Madison, Wisconsin, holds an annual festival billed as the World's Largest Brat Fest. The four-day charity event sees tens of thousands of brats sold by "celebrity" cashiers, usually local television, radio, and government personalities. Brat Fest's self-proclaimed world record is 189,432 brats consumed during the 2004 event.

Another town with German-American roots associated with bratwurst is Bucyrus, Ohio, which is known for its unique recipe incorporating fennel. It holds a bratwurst festival annually in mid-August attracting over 100,000 visitors annually. 

Bratwurst(s) is/are often simply called brat(s).

References

German food
Sausage