The Short Message Service (SMS) protocol is in widespread use among cell phone providers and allows users to send brief (160 western alphanumeric characters) messages to a phone number. The GSM hardware in the Neo1973 supports this mode.
The modem supports two message modes: PDU mode (AT+CMGF=0) and Simple text mode (AT+CMGF=1).
Protocol Description Unit (PDU) mode is an 8-bit octet schema, but the data is 7-bit. Information about SMS and the PDU format.
Text mode is simply a decoding of PDU mode within the firmware of the modem. The modem will guess the codepage based upon message contents and convert the PDU message to plain text. It does not handle multipart/mime or multimedia messages.
Some developers have created a python SMS Message sending tool. It posts commands to libgsmd-tool -m shell and reads the responses.
For more information, see GSM