when roaming, and wanting to restrict all data services to using wifi only, what the difference between turning off cellular data, and turning off data roaming ? To me they're basically both saying the same thing
Data roaming allows you to use data from other wireless providers when yours isn't available. Turning off data romaing simply turns that option off. With data roaming off... your iDevice will still pick up native data. So, lets say you're from the US and traveling to Mexico... where AT&T does work. However, the AT&T in Mexico is a different provider than AT&T in the US. You'll still be able to use data.... but be charged out the wazoo for it.