The iPhone connects to the Internet using two different methods: via Wi-Fi and via your cell network. While on Wi-Fi, you can use as much data as your Internet service provider allows, just as on a regular computer. On the cell network, however, all data you use counts toward your cellular data plan. Some plans offer unlimited data, but most have a cap and impose additional fees if you exceed it. To avoid these charges, be mindful of how you use the Internet on your phone and monitor your usage.
Checking Your Data Usage
The iPhone automatically tracks how much cellular data each app has used. On iOS 7, you can find this info by opening the "Settings" app and tapping "Cellular." The page lists the total data use since the last reset and breaks down the use by app. The phone does not know when or how often your cell plan resets your data limit, and so will never reset these numbers automatically -- you must do so manually by tapping "Reset Statistics." Alternatively, your cell carrier may offer an app that connects to your account and shows how much data you've used in the current billing period.
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Using Less Data
To cut back on data usage, avoid using the cellular connection to watch streaming video, download large apps or download podcasts. These activities quickly use up far more data than other tasks such as browsing the Web or reading email. To prevent accidental data use, you can turn off cellular data use on an app-by-app basis on the Cellular settings. If you're nearing your data limit for the month, you can also switch off "Cellular Data" to prevent the phone from using any data at all while not on Wi-Fi.