http://en.wikipedia.org/wiki/Switched-mode_power_supply

Strictly speaking the supply doesn’t push current, but the load draws it.

This means that you’re safe if the load draws less current than the PSU’s rated maximum, but it may blow it if you try to draw more. This can be spectacular and very smelly. Don’t ask how I know.

Loosely speaking, watts = volts x amps.

Also, if the output voltage of your little white cube (5 V, say) is less than the voltage required by the notebook’s charging circuitry (around 15 V for my Toshiba) nothing will happen or, at worst, the battery (rated at 10.8 V for the Toshiba) will try to discharge through the charger unless there is something (e.g., a diode) to prevent this.

It’s not the volts, it’s the wattage. Voltage stepdown is easy, adding the “push” or the current is the difficult part and usually requires a larger coil to translate the voltage into the current required for larger items. That’s soon to change with some very interesting I.C.’s from TI… Looks for wallet-sized laptop power supplies in the near future…