Dave is right. When you have equal characteristics fans(i.e equal head and flowrate) series arrangement gives you the capacity of a single fan but head gets doubled. Parallel arrangement gives you double the capacity of a single fan but head remains same.
Well, there was recently a thread where someone complained about water flow being used as an analogy for electricity. Well, here it is again: as others correctly said - parallel vs series configuration means more flow (amperage, flow) vs more pressure (voltage, head).
edit: If you internalize this concept (that energy transfers/flows of all types are related), you'll be waaaaay ahead of the curve as an engineer.