What makes a hot dog a hot dog?
Hot dogs are a an example indigenous to North America of a class of sausages called emulsified sausages.   The meat and fat are ground so finely that they emulsify together into a smooth paste.  Other sausages of this type include German Frankfurter Würstchen (of which the hot dog is a descendant) and Italian Mortadella (which is also the pre-cursor of the American Bologna sausage).

Hot dogs are cured, and typically hot smoked and sold fully cooked.

They can be made from a variety of meats, but beef and pork (or a mixture thereof) are the most common.

Hot dogs require a careful balance of lean (for the protein myosin, which provides the binding and structure to the sausage), fat, and water (usually added in the form of ice during the manufacture ring process, to help keep the nascent emulsion from breaking by keeping the mixture cool.)

There is no binding agent as such in hot dogs:  the myosin in the meet forms a matrix that provides the structure and chew of the sausage.  A hot dog without enough meat (or some other protein source) would not hold together.  This is much akin to the way a pâté is made--in some sense, a hot dog is a pâté stuffed into a casing, and hot smoked.

While it is possible to add fillers (and emulsify in additional water), it is not necessary and high quality manufacturers do so in moderation or not at all.

The major flavor elements in a New York style hot dog (where they originated) are typically the cured meat itself, salt, pepper, garlic, and paprika, as well as the smoky flavor.  Around the world, emulsified sausages are flavored with a wide variety of spices, especially warm spices, but these would not be typical in a hot dog.

You can find a recipe and slideshow with detailed pictures of the hot dog making process at Serious Eats.  It shows all of the key steps:  selection of ingredients with a careful balance of lean and fat, initial grinding, emulsification, stuffing, and smoking.