Automatic transmission

What is the difference between the gears you can put a car in? For
example you have 2, 3, 4 but what are they and when/why should you use
each one?

Yes it is a serious Q. Unusual I know for an adult but I speant hardly
any time around cars growing up so what seems obvious maybe to some is
not to me. I have gathered as much that certain gears should be used in
certain conditions like, going up/downhill, city traffic, etc but not
any detail.

Thanks in advance.

BTW can I assume D is 1 and also when should N be used?

If you have your manual, it does a good job of explaining the automatic gear
selections.
I assume that you have only driven manual? for towingD is normal drive gear,
N is Neutral for rolling the car out of gear, ex: being towed. The other
drive gears are for a lower ratio, primarily used for towing trailers over
and down very steep hills, not for normal driving at all. If you dont have a
manual, pop on in to your local Saturn dealer and ask if they have a spare
manual for your exact year and model.

marx404