The main difference is what level of power the hub can provide in total to the ports it has. A non-powered hub can only divide up the power that the single port it is plugged into is specified to provide- normally .5A@5VDC.
So in that scenario if you have several devices that have USB powering requirements, they will all be fighting for that one .5A@5VDC source meaning most or all will not work (particularly since you mentioned an external USB powered drive which sometime require a Y cable to obtain power from two ports on the computer all by itself).
A powered hub is able to provide the specified .5A@5VDC (or more) to each of the ports on the hub rather than dividing the single .5A@5VDC provided by the computer's port. A 4 port powered hub will have a power brick that is usually capable of supplying about 2.3A@5VDC (4x.5A+.3A reserve), and a 7 port hub brick is usually rated at 3.8A@5VDC (7x.5A+.3A reserve).
That way, each port is able to work with all USB devices no matter what their individual USB power requirements are. Non-powered ones are generally only good for low power requirement devices like keyboards and corded mice.
EDIT: BTW a powered hub also eliminates the draw on your battery so your working time is longer than if you hang the devices off a non-powered hub.