how an url shortening service works?

[gdatuk]

Any idea on the logic behind any URL shortening service like the tiny.mc, is.gd or snip.url?

every day some new service is cropping up but unable to find from google how it actually works.. any leads or ideas?

[Krazy Bluez]

It doesn't use any logic... it gives them a random number and alphabet unless provided by the user....