Usually, city water is a connection, using a water hose from the RV to a spigot at the campground. It works just like when you are hooked to water at your home, and there is no tank involved as it is a constant source from the city water supply.
Fresh water is a tank inside the RV, that you must fill with water separately, then you turn on the fresh water pump (operates off of 12 volts), which pumps the water from the tank to your taps. Most people only use the fresh water tank, when camping at places that don't have a water connection (aka dry camping)......or just don't like the campgrounds water and decided to bring their own.
Personally, I just keep a few gallons in the fresh water tank, so we can turn on the water pump and utilize the toilet if needed while traveling to/from the campground on long trips. It's real easy/fast to exit the interstate, use the onboard toilet, and get back on the road.
Here is a good article we keep in the FAQ section that explains the basics short and sweet:
RV Operations -The Basics
Welcome aboard, and feel free to ask any/all questions you may have.