Apps that require an Internet connection will use data on your plan or if you're on Wi-Fi it will use your Wi-Fi to connect, if the app doesn't need Internet, it won't use any data. If you are running things in the background that requires Internet, such as streaming radio or downloading podcasts. It may go to cellular. Hope it clears things up.
You will use data for a lot of things such as Siri, and maps.
GPS itself uses no data, but the apps that use GPS will use data. Mapping apps will use a lot of data, as the maps themselves are downloaded on demand in real time, and, with a couple of exceptions, are not stored on the device. Other apps that use location services can use a variable amount of data, depending on the app.
If you want to find what apps can use location services and which ones you have turned on. You may be surprised.
3. Location services
Click the various apps off and on as you wish.
CheersiPhone 4 (8GB), iOS 6.0.1
You need to check the specific location services app you want to use and see what it demands. As mentioned, those that stream live map data in real time, like Apple's own Maps app, need a constant data connection.
Strava does not - it will record the GPS coordinates of your route, and then you can upload the data to the web service once you are home and on wifi. Of course, if you want to lookup new routes and download those while out, it will need a data connection then (wifi or cellular). You can test that yourself by disabling wifi and cellular data while on your ride, with Strava running. When you get home, turn wifi back on and upload your ride's data.
One of the advantages of Tomtom and 3rd party GPS apps is that they keep all of the map data on the phone, so it doesn't need to be fetched over the air. This is also the disadvantage; Tomtom, for example, takes up 4 GB of your iPhone memory. They will still use small amounts of data to get your position, but not megabytes the way the built in or Google maps do.