SMS Short Message Service
Short message service (SMS) is a service available on most digital mobile phones that permits the sending of short messages (also known as text messages, or more colloquially texts or even txts) between mobile phones and other handheld devices. SMS was originally designed as part of the GSM digital mobile phone standard, but is now available on a wide range of networks, including 3G networks. -- 
SMS
MO SMS
MT SMS
SMS- Mobile-originated (MO) short messages are transported from the MO-capable handset to the SMSC and can be destined to other mobile subscribers or for subscribers on fixed networks such as paging networks or Internet protocol (IP) networks (including the Internet and private e-mail networks). Mobile-terminated (MT) short messages are transported from the SMSC to the handset and can be submitted to the SMSC by other mobile subscribers via MO-SM or by other sources such as voice-mail systems, paging networks, or operators.
- For MT-SM, a report is always returned to the SMSC either confirming the short message delivery to the handset or informing the SMSC of the short message delivery failure and identifying the reason for failure (cause code). Similarly, for MO-SM, a report is always returned to the handset either confirming the short message delivery to the SMSC or informing of delivery failure and identifying the reason.
-- from http://www.visualgsm.com/wire_sms_topic04.htm
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