Re: How do I use whatapp?
Simple version
When you send texts on your phone, you are charged your text rate according to your tarriff. When you text you are using the mobile phone network (2G/3G etc) to send the text.
Your phone also allows you to use the internet and Wifi spots when they are available. When you use the internet you are using your data allowance according to your tariff.
Whatsapp is an app that makes use of your internet connection and allows the exchanging of texts (chat) over that internet link. Because of that you do not get charged your standard text tariff price. Thus texting using WhatsApp appears to be effectively free. Of course if it's using your internet connection then it's eating into your internet data allowance but the theory is that the usage for texts is absolutely tiny so it is effectively free texting/chatting.
One must weigh up the security issues of texting using your mobile network vs using your internet connection of course.