What is a 'short url' and their associated risk?

The WIKI on the subject states:

URL shortening is a technique on the World Wide Web in which a Uniform Resource Locator (URL) may be made substantially shorter in length and still direct to the required page. This is achieved by using an HTTP Redirect on a domain name that is short, which links to the web page that has a long URL. For example, the URL http://en.wikipedia..../URL_shortening can be shortened to http://bit.ly/urlwiki or http://tinyurl.com/urlwiki. This is especially convenient for messaging technologies such as Twitter and Identi.ca, which severely limit the number of characters that may be used in a message. Short URLs allow otherwise long web addresses to be referred to in a tweet.

see http://en.wikipedia..../URL_shortening

In the example, one can clearly read the url and know it will take you to the wikipedia.org site.
The shorten form is unreadable (can't be understood).

So what you ask? Being unreadable means when you click on it, you have no assurity that you
will go where YOU intend, and thus can easily fall prey to a phishing scam and access some infected content. The HTTP Redirect can be deadly!