The first time you use the app in a new area (or the first time ever), it will need to download map data for that area. If you pan the map to look around, you will also download the data from that area.
Running the app in 3D mode uses much more data than in 2D mode since you need to see the map out towards the horizon.
That data is cached but will be replaced if/when changes are made to the map in that area.
Besides map data, your device is continuously sending data to the Waze servers to show where you are now and the traffic you are experiencing. Similarly, your device is continuously download data from the Waze servers to show you where other users are and what traffic they are experiencing.
Any searches you perform will use data as well as any routing requests.
Waze will use more data than a static GPS app, but it is doing so much more so it make sense.
Most people do not have an issue with quotas from what I have heard. I have unlimited data so I have never paid close attention personally.