what does cellular data do

Cellular data is Internet data that you receive from your phone provider eg. AT&T.

If you don't have cellular data included as part of your cell phone plan it is very expensive to use.

Switching it to off will stop your phone from connecting to the Internet via your phone service provider (potentially avoiding high charges) however you will still be able to connect via WiFi.

I have cellular data off but my phone provider has sent me a big bill. How is this possible?

Not having seen your bill, it's pretty difficult to say.

As kiltedtim said, without seeing the bill it's hard to say however I would guess that your contract with your phone provider includes a certain amount of cellular data (eg. 500Mb) as part of your plan and you are charged for this whether you use it or not. This is similar to how your phone provider may include say 500 mins of call time as part of your plan and you still get charged the contract amount if you only use 200 of the mins.

How does your Phone provider break down the bill?

So then turning off cellular data means you can go crazy with iphone and watch all the movies and play apps all you want,a long as you are connected to wifi and without charge??????

Wifi is not free. Somebody is paying for the data going through a wireless router.that you are going crazy with....

I have the same problem I got my bill little more than usual. So, I turned the cellular data off.