Twitter lets you write and read messages of up to 140 characters, or the very length of this sentence, including all punctuation and spaces. The messages are public and you decide what sort of messages you want to receive—Twitter being a recipient driven information network. In addition, you can send and receive Twitter messages, or tweets, equally well from your desktop or your mobile phone. There are a multitude of Twitter-related tools emerging all the time.
Twitter allows you to send and read other users updates (known as tweets)
Twitter messages(tweets) are limited to 140 characters (microblogging)
You can send and receive updates via the Twitter website, SMS(text messages), RSS (receive only), emails or a third party application.
You can restrict delivery to your circle of friends (delivery to everyone is the default).
You can use third party application such as Tweetie, Twitterrific, and Feedalizr to send Twitter messages.
You can search for people by name or user name, import friends from other networks, or invite friends via email. You can register at twitter sign up page and tweet!
You can learn more about Twitter at: http://business.twitter.com/twitter101/