What is an URL shortening service?
"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 especially convenient for messaging technologies that limit the number of characters that may be used in a message, such as Twitter, and for reducing the amount of typing required if the reader is copying a URL from a print source. Other uses of URL shortening are to "beautify" a link, track clicks, or disguise the underlying address."
Wikipedia