(Note: This topic was split from the LA board at: http://www.chowhound.com/topics/44114... -- The Chowhound Team).
Technically, XLB is a bun (or "bao").
Dumplings are, well, dumplings.
Wordsmithing aside, XLB are generally smaller than dumplings and are filled with "soup" -- meaning that the filling (either pork or crab or whatever) is usually made with some type of stock such as pork or vegetable. XLB are usually steamed.
Dumplings are generally larger in size, and are either boiled or pan-fried (although some can be steamed, but those are rare). The fillings for dumplings (whatever they might, pork, beef, seafood, etc.) are dryer in texture -- dryER, not dry -- not made with a significant amount of stock.
The skins on dumplings are also thicker in comparison to those of XLB. XLB skins should also be sort of translucent, and have a nice sheen or patina to them, esp. if they are steamed properly using good bamboo steamers.
The formation process is also a bit different. XLB are formed form bottom up, so that the bun is twisted and pinched to a close on top. Dumplings are formed more like a crescent moon and are not pinched, but rather should be formed by pleats (if that makes any sense at all ...)
Places of origin also differ. The XLB you have in mind, i.e. soup dumplings, are Shanghainese in origin. Dumplings are generally from northern China, think Beijing and the like.
Hope that helps.