It is a communications protocol allowing the interchange of short text messages between mobile telephone devices.
The SMS technology has facilitated the development and growth of text messaging. The connection between the phenomenon of text messaging and the underlying technology is so great that in parts of the world the term "SMS" is used colloquially as a synonym for a text message from another person or the act of sending a text message.

SMS as used on modern handsets was originally defined as part of the GSM series of standards in 1985
[1] as a means of sending messages of up to 160 characters, to and from GSM mobile handsets.
[2] Since then, support for the service has expanded to include alternative mobile standards such as ANSI CDMA networks and Digital AMPS, as well as satellite and landline networks.
[3] Most SMS messages are mobile-to-mobile text messages, though the standard supports other types of broadcast messaging as well.

Typically a text message originating at a handset is sent to a Short Message Service Centre (SMSC). The SMSC then attempts to send the message to its recipient. If the recipient is not reachable, the SMSC queues the message for later retry. This mechanism is characterized as a store-and-forward delivery system.

SMS message transmission is also characterized as best effort: there are no guarantees that a message will actually be delivered to its recipient, and delay or complete loss of a message is not ruled out by the specification of the protocol.

SMS messages can be used for text communication between mobile phone users, for carrying queries and responses between the phone user and computerized information services, or by the network operator for various management tasks. A variety of technologies have been developed to allow SMS messages to be interchanged with other networks, e.g. analogue phone lines or the Internet (websites, email etc).