The location indicator in the top bar of iOS seems to have two states: hollow and solid.
I know that the icon is visible whenever an application is using Location Services, what's the difference hollow and solid?
The hollow location indicator is used to show that one or multiple applications have active geofences. For example, you can use Reminders.app to remind you of something “when you get home”. To detect when you are near your home location it has to check your location every now and then to compare it to your home. This will usually happen when you unlock your phone (indicator goes solid for a moment), when your GSM chip hands over to a different cell and when you associate with a wifi network (WPS). This icon is shown so you know that location services are activated at irregular intervals which may affect your battery life.
The solid indicator means it is actively locating your device. You will notice, after opening the Maps.app, for example, that the indicator turns solid and remains solid. This is because Maps.app wants your exact location constantly. For a few other examples, when you start Siri, this indicator will also appear, as well Local Weather, or the Camera app (if geo-tagging has been left on).