That's sneaky. How does one do that?
It is meant for changing HUGE urls to something more manageable (for posting, sending over email, sending over instant messengers, etc).
See also http://snipurl.com/ for another one.
These services are unfortunately now commonly abused to send unsuspecting people to “shock sites” (generally animal or gay porn, or something equally revolting) in a way which cannot be anticipated (unlike Meta refresh tags, which you can examine in the html source before clicking).
There is a ‘trusted’ version where, for the price of a 2nd click, you can get a preview of the tinyurl: for example, click http://preview.tinyurl.com/33wvbv and you will see that the link refers to FR before you actually visit FR.
Anyway, the idea is that, for example, someone (usually an idiot kid) will find an FBI link and spam it on message boards hidden as a tinyurl, give it a safe description and safe context, and laugh as the FBI stomps grandma’s *** with a full tactical entry. The kid does not even have to click it himself.