TinyURL
TinyURL is the most popular service providing short aliases to long URLs, and also functions as a general term for such services.
Such services consist of two parts.
One is a web page with a text box for writing long URL, submit button, and some text describing the service. When user inputs a long URL, for example http://en.wikipedia.org/wiki/Special:Recentchanges, the server adds a new alias in its database and returns a short URL, in that case http://tinyurl.com/24xz9. Because users often ask for the same URLs, the service may also return an already existing alias.
The second part is a forwarder. When the web browser asks their server for http://tinyurl.com/24xz9, it will be forwarded to the full URL.
Short URL aliases are useful because they're easier to write down, remember or pass around, are less error-prone to write, and also fit extremely bandwidth-limited media, like IRC channel topics or email signatures.
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