URL shortening is a type of internet service that provides a URL which is redirected to a page with a longer and more complex URL. A company that provides such a service is a URL shortener. Shortening a URL is useful on posts to Twitter and other services that allow a limited amount of characters. It can also be useful for cleaning up the appearance of some URLs, which can be very long when they contain a large number of parameters. Some shorteners also provide some analytics on their links such as counting the number of times it is clicked, which can be useful for tracking the spread and popularity of links.
URL shorteners often have short nonstandard top level domains, for example, bit.ly. The .ly suffix is the country code for Libya, in this case used because it is one letter shorter and provides for a continuous company name.