what Andy says is of course correct...
heres another way to look at it (assuming we are talking of 'paired' batteries of the the same group size and voltage):
SERIES connection is when you hook the POS on one battery to the NEG on the 2nd battery - the battery VOLTAGE will be DOUBLED and the AMP HOUR rating will be the same as the rating of one of the batteries.
PARALLEL connection is when you hook the POS of one battery to the POS of the 2nd battery, and likewise, the NEG terminals are hooked to each other - the battery VOLTAGE will be the same as one of the batteries, and the AMP HOUR rating will DOUBLE the rating of one of the batteries.
6 volt - SERIES - 6 volt = 12 volts
- PARALLEL - 12 volt
= 12 volts
another often overlooked situation concerning PARALLEL battery systems, is to try and hook the LOAD CABLE (POS) to one of the batteries - and - hook the GROUND CABLE (NEG) to the OTHER battery...some of our AS's don't really allow this type of connecting due to battery boxes being separated from each other - but if you have your batteries mounted beside each other, try to hook LOAD and GROUND cables to alternate batteries...
There is a slight inherit RESISTANCE in each cable connection, and by hooking the POS & NEG supply cables to alternate batteries, you tend to EVEN OUT this RESISTANCE to help insure both batteries are charged and discharged evenly...