Booster nailed it (as usual) but I thought I'd add the numbers for you.
When the batteries are in parallel, they provide the same output voltage, but the Ampere-Hour ratings get added together. Example, two12v batteries with a 90 aH rating give you 12v at 180 aH. That goes to how long they will last.
Conversely, in my rig I have two 6v 235 aH (golf cart) batteries in series, which makes the voltages add together (for 12v total) at the same 235 aH.
Golf cart batteries make good house batteries because they generally have higher aH and are designed for deep cycle operation. Typical car batteries are designed to manage peak outputs, represented by their Cold Cranking Amps (CCA) rating.
Booster did I say it right?