Upcoming travel questions for my iPad 3

Hello All,

I did a search before asking but all I could find were questions about travel adapters.

I have an iphone 5 and an ipad 3 and I am traveling to the Netherlands and Budapest. I'm just a little confused about what I can and can't use while gone that won't cost me an arm and a leg. I will add the AT&T world traveler program for while I'm gone for phone calls but I'm confused about wi-fi. If I'm in a wifi area will I be able to text/imessage people without a charge? What about searching the interenet or reading email?

I get a little confused with what to turn off on the devices to make sure I'm not getting roaming charges etc. I need to turn off the 4g on both devices correct?

Please forgive the naive questions, I don't generally travel outside the US and I'm trying to plan ahead.

Thanks in advance!

Stephanie