phranque - 9:05 am on Oct 4, 2010 (gmt 0)

typically the server-side script or the server configuration is designed to recognize the user agent as a mobile browser and responds to the requested url with a redirect to the equivalent mobile url.

this w3c document gets pretty geeky on the subject.

Delivery Context Overview for Device Independence:
http://www.w3.org/TR/2006/NOTE-di-dco-20060320/ [w3.org]

you might also do some searches on [mobile content negotiation] and [mobile user agent detection].