Generally, if not said otherwise, in context of portable power banks for cellphones and tablets, it is virtually always USB.
In rare cases there can be also a 9 or 12 volt output. 19 volts in case of a power bank for a laptop, but these are quite rare and way more likely to specify the output voltage explicitly, or at least obliquely (the 12V ones often advertise ability to recharge car battery to assist with cranking up).
