The above video explains the concept neatly.If you dont have the time here is the short version each person using the app helps update the app passively yes you just have to start the app when you drive and the waze servers do the hardwork for you updating the map and warning you if their is some obstruction ahead.
The map is completely user generated and you can reprot accidents,traffic jam and login into foursquare.If you find a road is not there you have three options
- report the problem
- press record on the app and start driving
- login to www.waze.com to change the map
Download the app from ovistore http://store.ovi.com/content/16466