Buns are small, hand-sized, sweetened bakery items usually made from a flour, often with a leavener, either yeast or a chemical leavener.
They are usually dome-shaped, with a flat bottom, and may or may not be iced. They are usually round, though, they can be longish instead.
The word “Bun” now has many definitions to it. In North America, people would be hard-pressed to tell you what the difference is between a bun and a roll, with the bottom line being that the difference often just comes down to common usage for a particular item for that region.
The Oregon State Statutes reflect the confusion: “As used in this subsection, ‘rolls” or “buns” include doughnuts, sweet rolls or sweet buns made with fillings or coatings, such as cinnamon, the soft rolls, such as Parker House rolls, hamburger buns, hot dog buns and the hard rolls, such as Vienna rolls or Kaiser rolls. However, rolls or buns shall not include foods made with specialty flours, such as cake flour.” 
In Newfoundland, Pork Buns are cross-category items: they have salt pork in them, but in addition to the pork, can also have raisins and molasses.
By “Chinese English”, we mean how Chinese food item terms get translated into English. Most small, flour-based (wheat or rice flour) Chinese items get translated just as “buns”, whether they are savoury or sweet — and bear in mind that as an ancient cuisine, Chinese cooking still blurs the boundaries between sweet and sour to start with.
The word in Chinese for bun is “bao.”
Steamed Buns (such as “baozi” or “mantou”) can be sweet or savoury, filled or unfilled. “Xiao Long Bao”, dumplings with a filling and a small amount of broth inside, are sometimes translated in English as “Juicy steamed buns with pork.”
One thing is for certain now about Chinese “buns.” In January 2008, new national laws came into effect in China stating that Steamed Buns had to be circular or crescent shaped, to the outrage of those who made square-shaped buns.
 Oregon State Statutes 625.212 Definitions for ORS 625.215.