Today when messaging is main-stream because of mobile applications like WhatsApp, Viber, Facebook Messenger, LINE and many more, offering free messaging over internet (from 2G to 4G, or your wifi). Before this to sent a SMS you was limited to 160 character per message, and it's cost is almost equal to cost of normal calling charges. But with internet messaging apps you can chat for free (you just have to pay for data connection, again which cost is too too less compared to standard message rates), which are not even limited to 160 character.
But even to use these messaging apps you must have an active internet connection either cellular network or wi-fi network to transfer messages. Here comes the "off-the-grid" way to chat - FireChat; chat without internet connection. Yes even no cellular connection is required, not even mobile phone coverage.