wattage is the product of current (measured in amps) multiplied by voltage (measured in volts),

wattage is power, energy expended over a period of time, 1 watt is 1 joule/sec

mAh (milliamperehours) and Wh (watthours) are a measure of the energy capacity of a battery, how much total juice a battery can hold when fully charged

I think you are correct about chargers. You probably want the charger to be capable of more power than the device needs. Also look out for this, https://upload.wikimedia.org/wikipedia/commons/6/68/AC_adaptor_polarity.png because I've blown up circuit boards by getting the polarity wrong.

also, voltage is roughly constant with electronics and the current is what changes when more or less power is used.