What is navigation? What qualities would a good navigation system have? What would make a navigation system ineffective?

Navigation is a way of guiding. For example, a car has a nagigation. People can buy a navigational system put into their cars for directions on where they want to go and how to get there. Good qualities would be.....
1.)Simple navigation.
2.)Readability.
3.)User-friendly.
Without these things, a navigation system would not be effective.