The only restriction that twitter had since its inception was of 140 characters message limit.
But recently there has been a flourish of online applications and browser add-on that let you post the tweets with more than 140 characters.
Basically this is how this system works:
After login to Twitter, you put down your text in twitter update box with more than 140 characters and notify the application that you are ready with your message.
The application reads your text shortens it to fit 140 window and url is provided to read the excess message.
While some applications allows to break the message of more than 140 character into mutliple parts and update in form of multiple tweets on twitter client; though this method is less comparatively famous.
Some online web applications that provides you such facility are:
Twitlonger:
Gives you the bigger window to put your text and update the message url to Twitter client.
Twitzer:
Twitzer is a Firefox extension which lets you post text longer than 140 characters on Twitter.
Bigtweet:
Allows the message update in parts, if more than 140 characters.
You might want to take a look at Runt.ly http://runt.ly It goes beyond BigTweet, et. al by allowing you to create short blog-like posts, with pics and embedded videos, that can shared on Twitter with single short url.
TinyPost is another service that allows you to post long messages on Twitter. Try it http://tinypo.st