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URL shortening

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 a redirect on a domain name that is short, which links to the web page that has a long URL. For example, the URL "https://en.wikipedia.org/wiki/URL_shortening" can be shortened to "http://bajaurl.com/1tjXqrr".
This is especially convenient for messaging technologies that limit the number of characters that may be used in a message, such as SMS, and for reducing the amount of typing required if the reader is copying a URL from a print source. In November 2009, the shortened links of the URL shortening service Bitly were accessed 2.1 billion times.

Other uses of URL shortening are to "beautify" a link, track clicks, or disguise the underlying address.
Although disguising of the underlying address may be desired for legitimate business or personal reasons, it is open to abuse and for this reason, some URL shortening service providers have found themselves on spam blacklists, because of the use of their redirect services by sites trying to bypass those very same blacklists.

Some websites, such as is.gd, prevent short, redirected URLs from being posted.
Source: Wikipedia