Biztalk supports both Hub/Spoke and ESB(provided as ESB Toolkit). Conventionally Biztalk is used as a Hub/Spoke model where the Biztalk Server acts as a Hub and adapters connecting to the Biztalk Servers acting as a Spoke.
In Terms of typical definition , Hub/Spoke takes care of all the processing load in the Server side whereas in ESB Model the application load is taken care in the connecting applications(Client).
Do the client need to have adapters to communicate to the Biztalk ESB application and does the processing load fall on the client side ?