What constitutes Date Usage on an iPhone 5?
You need to contact your Carrier, i.e. Verizon, AT&amp;T, Sprint etc, to see what they define as Data Usage.

That said, generally speaking, any communication that you send/receive/request over their network while directly connected to their network, and while Roaming, is typically counted under Data Usage.  This could be emails, websites, pictures, videos, etc. Typically when connected to a Wi-Fi Access Point then Data Usage Accounting is not counted against your monthly Data Usage allotment.