what is the difference between "cellular data" and "data roaming"?

If you have Data Roaming turned off, you won't be able to access the internet at all over the cellphone network whilst not connected to the network your SIM is designed for.

Having Data Roaming turned off will have no effect if you are connected directly to your SIMs cell network in your Home country.

If you turn off "Cellular Data" you won't be able to access the internet at all, regardless of which cell network you are connected to.

Would there be any adverse affect to my bill if I have "cellular data" on while outside of the US?

If you have Cellular Data turned "on" whilst outside the US, but Data Roaming "off", then you still won't be able to access the internet as the Data Roaming "off" setting would override the Cellular Data "on" setting.

See: http://support.apple.com/kb/HT4203

Data Roaming is when you are connected to a network other than the one your SIM is for, often in another country.

Cellular Data is just data over the cellphone network, any network, regardless of country or SIM.

inte wrote:
What data is filtered by data roaming vs/cellular data? For example, email, calendar, contacts, text messages etc... are all or some updated by only one or the other? I'm just trying to understand exactly what the two terms mean as far as what they do to the phone.

They function exactly the same: both will cut off ALL data that isn't done over WiFi. The difference is that 'Cellular Data' always does this, while 'Data Roaming' only does this when you are not on your home network (presumably AT&T, though I do not know where you are from.)

As such, turning both off while abroad is actually redundant.

This is not true.

You absolutely can access wifi with data roaming turned off. While overseas, I would turn off cellular data too since most providers overseas charge for data at high rates.

I know this for a fact because when I went to Spain with my 3G, I turned off data roaming and used the phone to access the internet via wifi.

Data roaming is merely the phone's ability to find another provider for data if your phone's provider is not available. Similar to your cellular phone roaming to make a call thru another provider.

I would switch both off.

I'm travelling in the USA from the UK with my new iPhone 4 and even with data roaming off, I received a visual voicemail message which appeared to then trigger the downloading of 25 e-mails and enabled selective web browsing and app activity.

I'm going to call my network operator tomorrow, but others have complained about this bug (that first appeared with the 3G iPhone and was fixed by an update) and they all recommend turning off Cellular Data as the only way to gaurantee no data roaming.