If using WiFi, you will only be charged anything if the establishment providing the WiFi access charges. If they do, you wouldn't be able to use it without paying them. If your hotel, nearest Starbucks or McDonalds provides free access to Wifi for customers then there is no charge. You can do anything over WiFi abroad that you can at home, including using iMessage and FaceTime, email and web browsing.
Make sure 3G/4G data / Cellular data is turned off in your phone to ensure you get no roaming data charges.