Great explanation. Thanks.Twitter allows 140 characters. If your URL itself is taking much characters, like this, there wont be much space available to write.
http://www.hostingdiscussion.com/website-development-design/20475-tinyurl-twitter.html#post106813 (97 characters) is now
http://************/oks4ck (25 chars)
There are other URL shortening services which does it below 20.
Personally, I don't click on tinyurl links unless I know the person who provided the link. You can shorten a link by other means that make it more trust worthy, like using php.