Twitter

Twitter is a free social networking microblogging service that allows registered members to broadcast short posts called tweets. Twitter members can broadcast tweets and follow other users' tweets by using multiple platforms and devices. Tweets and replies to tweets can be sent by cell phone text message, desktop client or by posting at the Twitter.com website. 

The default settings for Twitter are public. Unlike Facebook or LinkedIn, where members need to approve social connections, anyone can follow anyone on publicTwitter. To weave tweets into a conversation thread or connect them to a general topic, members can add hashtags to a keyword in their post. The hashtag, which acts like a meta tag, is expressed as #keyword. 

Tweets, which may include hyperlinks, are limited to 140 characters, due to the constraints of Twitter's Short Message Service (SMS) delivery system. Because tweets can be delivered to followers in real time, they might seem like instant messages to the novice user. But unlike IMs that disappear when the user closes the application, tweets are also posted on the Twitter website. They are permanent, they are searchable and they are public. Anyone can search tweets on Twitter, whether they are a member or not. (Source: Whatis.com)