URLs can be monstrously long. An Amazon URL, for example, can wrap for three lines of text. The problem with long URLs is that sometimes they jut don't perform properly when clicked. So, a service called TinyURL allows you to enter that really long URL and come back with something considerably shorter. Unfortunately, phishers have discovered this little trick, too.
Phishers have started using Tiny URL as means of hiding their fake URLs from people who receive their phishing messages. Instead of seeing the long (and bogus) URL, even when they mouse over it in a message, all users see is: http://tinyurl.com/aa4os8.
Not only does the URL hide the actual location of the web page linked to (until the user is on the site), but it also makes it easier to get past spam filters, which means more phishing messages are making it to people's inbox. So use caution when any URL shows up in your email messages. Don't click through the link unless you know the sender and were expecting the link.