When you're at home, work, or a friend's house, it's always best to connect to their wi-fi network since it's faster, more secure and will save you from using cellular data.
When you're in your car, traveling, or just out and about, you'll likely be using cellular data. Which if you're like me, you monitor since your Cellular Provider charges you for overages.
Here's how to quickly tell if your iPhone is connected to Wi-Fi or is using Cellular Data.
Looking at the status bar, if you see the Wi-Fi Symbol, you're connected to a Wi-Fi network.
If you see LTE, 3G or 4G your iPhone is using Cellular Data.