Snatch App uses your phone’s Internet connection (4G/3G/2G/EDGE or Wi-Fi, as available) to send and receive messages to your friends and family. You do not have to pay for every message. As long as you have not exceeded your data limit or you are connected to a free Wi-Fi network, your carrier should not charge you extra for messaging over Snatch App.
Snatch App Calling works in the same way.

Please be aware that:

If your phone is roaming, additional mobile data charges may apply.
If you send SMS messages to your friends inviting them to use Snatch App, service charges from your mobile provider may also apply