Hi Kenny,
Well, most of these boats have an engine cranking battery which is separate from the battery(s) for the radio, cabin lights, etc..
The battery set that is not (normally) used for cranking the engine is referred to as 'house batteries' - as a shorthand form of saying, 'the batteries not normally used for cranking'... There may or may-not be a switch to put the house batteries onto the engine circuit for cranking if the engine battery is discharged...
As the boats get shorter it is more common to have a single battery (or pair of batteries in parallel) for both engine cranking and running the sounder, lights etc... My 14 foot runabout has a single battery for engine, radio, lights...
In my boat I have a pair of Group 27 batteries as the house batteries, another Group 27 as a cranking battery, and another as the anchor winch battery... There are switches to keep the house battery and cranking battery separated or to put them in parallel...
Group # is a measure of battery physical size, and the bigger the battery the more amp-hours it has - usually... SO a Group 31 is bigger than a 27, and a 21 is smaller than a 27, etc...
As I noted, I am adding a pair of 6V golf cart batteries (in series for 12v) in the starboard lazarette... I have a switch to separate them from the house bank, so that I will have two sets of house batteries so I can limit discharge on a bank to 50% and then switch to the other bank... If I have said anything confusing, don't hesitate to ask for clarification..
cheers...