The app uses maps and your phone's GPS. Map information that is already available cached in your phone and information that is retrieved from the Waze map servers via the internet. It is possible to use Waze without internet connection while driving in areas where your phone uses already cached maps, but the best navigation experience you got with an internet connection. The data usage of the app is moderate. The test of the ANWB mentions 3,9 MB for 75 KM.