Hi Mills and welcome to the forums!
GPS navigation can actually work with Wifi, mobile data or technically no data at all (more on that in a minute.)
While driving, if you have data enabled (Settings > Data manager > Data delivery) then you can continue to drive and use navigation via your mobile data plan with your carrier. Be careful how much you use it as it does charge data against your account. Google Maps/Navigation now has the ability to cache up to a 10 mile radius of maps in case you lose the data connection while driving.
That brings me to GPS navigation without data or Wifi. Certain GPS navigation apps offer the ability to download the entire US map (or other countries if needed) to the phone and then navigate using only the GPS receiver within your phone. All you have to do is make sure the GPS is enabled (Settings > Location and security > Use GPS satelites.)
Possibly take a look on the Market if you're in the need for one of the pre-download map apps.