Twitter is a service for friends, family, and co–workers to communicate and stay connected through the exchange of quick, frequent answers to one simple question: What are you doing?
You can use 140 characters or less to send your thoughts, observations, and goings-on in your day. Whether you’re “eating an apple” or “looking foward to the weekend” or “Heading out of town”, it’s twitter-worthy. Twitter messages are called tweets.

Hashtags are a community-driven convention for adding additional context and metadata to the tweets. An online hashtag is created by prefixing a word with a hash symbol: #hashtag.

Followers (followers_me) are those who have elected to receive tweets from a twitter, following_me are friends whose tweets a twitter is following.

Twitter can be used on the web or with a mobile phone (SMS), it’s interworking with Facebook and it’s supporting an API to develop specific applications. Twitter offers an search and advanced search webpage to find out what’s happening in the world beyond your personal timeline. To search, filter, and otherwise interact with the volumes of news and information being transmitted to Twitter every second, Twitter Search helps to filter all the real-time information coursing through their service.

Twitter is a privately funded startup with offices in the SoMA neighborhood of San Francisco, CA. Started as a side project in March of 2006, Twitter has grown into a real-time short messaging service that works over multiple networks and devices.

Detailed informations are available on Wikipedia.