Voltage is very roughly speaking the push/force applied to the electrons. Wiring the batteries in series increases the voltage (push). It might help to think of a battery as something like a little pump, and voltage is the pressure it produces. Putting two pumps in series allows you to get twice the pressure.
Current (amps) is a measure of how much electricity is flowing (or how fast it goes). It can be defined as a rate of electrons passing a given point.
Amp-hours, and milliamp-hours, are measures of charge—a quantity of electrons. If we were talking about water rather than electrons, they'd be something like gallons.
Watts are a measure of power, and watt-hours a measure of energy. Wattage is found by multiplying voltage times current, so one watt is the power of one amp at a voltage of one volt. Watt-hours, the measure of energy, can be found by multiplying amp-hours by voltage.
Resistance is a separate thing altogether; it's a measure of the opposition to an electric current. The resistance in a theoretical circuit represents the energy being consumed by something, whether usefully or otherwise. By definition, a current of one amp will flow when there's a voltage of one volt across a resistance of one ohm—that's the essence of Ohm's law. A camera does not represent a fixed resistance; its sometimes uses a lot of power (high current), and sometimes very little power (low current). The camera does need a more or less constant voltage to operate, hence the use of six AAs or two Li-ion cells to produce a nominal 7.2 V supply. (Internally, this is regulated and manipulated to produce a bunch of different voltages that the different camera parts need—possibly 3.3V or something similar for much of the digital electronics, possibly 5V for the LCD backlighting, several hundred volts for the flash strobe, and various others.)
And I got the capacities the wrong way round.
