What is the difference between mobile data and data roaming?

When you are at home you do not roam. You will be using data on your plan = mobile or cellular data over your carrier's network. If yo are in another country and you have roaming set up to make calls ( this is done thru your carrier before you leave home) you will be able to make and receive calls overseas at an expesnive price. While overseas if you have data roaming on you will be able to surf the net and do other things like send email etc using data over the cellular network of whichever carrier you hook up to overseas. THIS IS VERY EXPENSIVE When overseas get a local sim card if your phone is unlocked and use that. You should avoid data roaming if at all possible

I would turn both data roaming and mobile data off. If you are really concerned and only want to use wifi and are not willin got chance anything you could turn on airplane mode and then just turn wifi back on. that way if you find a wifi network you could just use that to send imessages and emails etc for free (if the wifi network is a free one that is)