SMS Gateway

An SMS gateway allows a computer to send or receive Short Message Service (SMS) transmissions to or from a telecommunications network. Most messages are eventually routed into the mobile phone networks. Many SMS gateways support media conversion from email and other formats.

Evolution of SMS Gateways
At one point, SMS Gateways were actual physical pieces of hardware with SIM cards and embedded radios. Each SMS Gateway was individually connected to mobile phone networks to send text messages in a very similar way to a regular cellular phone.
Nowadays, modern SMS Gateways usually route SMS text messages to the telco networks via an SMPP interface that networks expose, either directly or via an aggregator that sells messages to multiple networks.
SMPP, or Short Message Peer-to-Peer, is a protocol for exchanging SMS messages between Short Message Service Centers (SMSCs) and/or External Short Messaging Entities (ESMEs).
Computers can interact with SMS Gateways in multiple ways:
Through a REST API, where software developers send and receive SMS text messages over HTTP to the REST API, like Twilio Programmable SMS
Using a Web Page or Software, where customers upload their messages and list of phone numbers
Refrence

A short message is a brief text message sent to or from a mobile phone subscriber through the Short Message Service (SMS). The standard short message consists of up to 160 alphanumeric characters, although messages at least 50% longer can be sent using data compression. Developed as part of the Global System for Mobile communications (GSM) Phase 1 standard, a short message is exchanged between two mobile devices or between a nonmobile device and a mobile device (for example, a short message can be sent from a PC attached to the Internet to a mobile subscriber). Short messages are stored in and forwarded from a Short Message Service Center (SMSC) so that - unlike the user of a pager - the recipient can get messages that arrive when their mobile device is not turned on.
SMS compression increases the amount of text that can be sent, and SMS concatenation enables short messages to be strung together into a longer one. Users of devices that are not SMS-enabled can send short messages using an alternate version known as Internet SMS.
There are many companies in the Indian market who rent keywords, on a monthly basis, whose characters, on a typical telephone keypad, represent short codes. Short codes are five digits in length and have to start with the digit '5'. The five digits can be extended by three digits further representing three additional characters. Texts sent to these Short Codes are commonly referred to as Premium Rate SMS Messages and cost around Rs 1 to Rs 3 per text depending on the operator as well as the service. Any length of full message can be sent, ranging from 100–500 (some providers only support).
2) Instant messaging (IM) messages are also sometimes referred to as short messages. To make the most of a short message, people frequently use a shorthand typing mixture of letters and numerals known as Alphanumerish.
Reference