In the phrase "SMS messaging," "SMS" stands for "short message service." SMS is a means of sending text between mobile phones, and it can also be used to send text from a computer or tablet to a mobile phone. Only 160 characters are allowed in an SMS message.
Many cell phones also allow MMS messages, which permits longer strings of content and allows for the sending of pictures. SMS messages are not sent along the voice channels that allow for phone calls. Instead, they use the control channel which allows a mobile phone to contact the local cell phone tower. This channel is also used to send a ringtone to a phone.