WhatsApp allows you to send free text messages over the internet, saving you the cost of sending SMS.
You can use WhatsApp to send messages over WiFi or 3G to anyone else with the app installed on their phone. The software works across mobile operating systems (iPhone, Android, BlackBerry, and Nokia) so it will function with contacts who have other versions of WhatsApp installed.
Once set up, WhatsApp is pretty easy to use. Just type and hit Send to post the message. WhatsApp gives you a number of other handy options aside from just sending text. You can attach a picture (taken from your camera or stored on your device), audio, and video. There's also an option to email your chat history.
The Windows Phone version of WhatsApp is a bit more basic than it is on other mobile operating systems. For example, it doesn't save conversation histories and notifications don't work well.
Remember that WhatsApp only works if both people in the conversation have the app installed.
Overall though, WhatsApp makes for a handy way to chat for free on your Windows Phone device.