URL shorteners are a very handy tool. Let's say you want to send a very link to a friend. For example a link to a certain ebay item. usually very long links a broken into two parts in an email.
On the webpage of URL shorteners you can enter that long link (cut'n paste) and it will be transfered into a very short one. Some of them (like tinyurl.com) also offer a bookmarklet so you don't have to enter the link manually.
