A hotdog bun is a type of soft bun shaped specifically to contain a hotdog. There are two basic types: top-loading, which is popular in New England, and side-loading, preferred in the South and Midwest United States.
The advantages to a top loader are that it holds the hotdog securely and fits nicely into little three-sided paper boxes. Top loaders are generally baked side by side and torn apart as needed, leaving a flat side surface for grilling.
Side loaders tend to be doughier, so are more likely to successfully sop up all the juices from chili or sauerkraut without falling apart.
Other regional variations include additions such as sesame seeds or poppy seeds. In Chicago, Illinois, where poppy-seed buns are popularly served with Chicago-style hotdogs, the buns are made with high-gluten flour to hold up to steaming.