Definition #2
from "Twitter a Status Symbol on the Web"

1.Twitter
Twitter is a social networking site, which provides micro-blogging service. People can send text-based posts of up to 140 bytes in length. It can be sent and received updates via the Twitter website, SMS, RSS (receive only), or though applications such a Tweetie.

2.Micro-blogging
It is a form of multimedia blogging that allows users to send brief text updates or micromedia such as photos or audio clips and publish them, either to be viewed by anyone or by a restricted group which can be chosen by the user.

3.SMS
Short Message Service (SMS) is a communication service standardized in the GSM mobile communication system, using standardized communications protocols allowing the interchange of short text messages between mobile telephone devices.

4.Smartphones
A smartphone is a mobile phone offering advanced capabilities, often with PC-like functionality. There is no industry standard definition of a smartphone. For some, a smartphone is a phone that runs complete operating system software providing a standardized interface and platform for application developers.

5.Social media
It is information content created by people using highly accessible and scalable publishing technologies that is intended to facilitate communications, influence and interaction with peers and with public audiences, typically via the Internet and mobile communications networks.