What Is iMessage and How Do I Use It?

You can send unlimited free text messages from any iOS device to any iOS device over Wifi or 3G. Technically, you can also send pictures, movies, etc. without being charged but keep in mind that if you are using 3G some data fees may apply (this pretty much depends on your data contract).
Pretty much the coolest new feature of iOS 5, coming this fall. iMessages will forever change how you send texts.