For example:
The USB Block Erupters from ASICMiner are powered via USB. They don't have an alternative DC power source like the Gridseed does. Daisy chaining multiple unpowered USB hubs will add more USB ports to your computer, but your computer can still only supply a low amount of power for all the ports on the USB hub. Powered USB hubs fix this issue. (i.e. Plugging in multiple, and daisy chainning, surge protectors and extension cords to one outlet in your house doesn't supply more power. It only allows more things to be plugged in. Everything would still be running from the one circuit breaker on your house's electrical box.)
Gridseed uses USB for the purpose of data between the unit and computer, and gets it's power from an adapter/PSU. ASICMiner Block Erupters use USB for both the data and the power (in the similar manner you'd sync and charge an iPod at the same time).
Hope that clears it up some.
That was a nice explanation man!
Good stuff