Fish dude, this is quite a hard thing to explain depending on how much you know.
Basically:
Voltage is a sort of push (it's joules/coloumb of electrons) and you can forget this and pretend it's 12v constantly from a battery* If this was though of in a context of flowing water down a pipe, this is how quickly it would be pushed down really.
The number of amps is the amount of current coming out - so in this case it's like the amount of water being pushed by the voltage.
Generally a high voltage leads to a high current flowing. Power = current x voltage, so you get a higher power output.
For car batteries you get a rating like 50Ahr
For example, 50AmpHrs means that it can provide 12volts at 1amp for 50 hours or any combination (i.e. 2amps for 25 hours at 12volts). This would mean 12watts are being used (watts are power and power = voltage x current).
It gets more complicated however in that some batteries are intended to provide the power over a long period of time but at a small current where as others are intended to provide a lot of power for a short period of time.
The higher the AmpHr rating of your battery the better but you also need one suited to a car application.
Sorry this is very broken up and not very clear, I just realised how hard it is to explain for me.
* This isn't true because car batteries have an internal resistance (this is why your lights dim when you start up the car, not that the battery can't provide enough power like most people think) - you don't need to know about that though.
