12 Volt-Direct Current (DC) 12 volt is the type of electricity you would find in an automobile. Your battery is the foundation for the 12 volt system in your RV. A fully charged battery will operate the 12 volt equipment (furnace, water heater, most lighting, range hood, monitor panel, roof vent fans, etc.) in your unit until the battery becomes discharged. The level of discharge for operation to cease will vary by each piece of equipment.
120 Volt-Alternating Current (AC) 120 volt is the same type of electricity you typically find in a home. Plugging your shoreline cord into a power source (campground or other appropriate receptacle) is the foundation for the 120 volt system in your unit (optional generators produce 120 volt electricity as well). Certain equipment (air conditioner, microwave, most televisions, etc) in your RV can only operate from 120 volt electricity thus require the unit be plugged in to a power source or a generator running.
Your unit is also equipped with a “Converter.” When the shoreline cord is plugged in at a campground (or other appropriate receptacle), this device “Converts” 120 volt electricity into 12 volt electricity. Most 12 volt equipment in your unit will operate from this power source, even if your battery is discharged. The “converter” is also equipped with a “battery charger” which provides a “trickle charge” (slow, low amperage charging) to your battery as long as the shoreline is plugged in to a power source.