There’s a growing trend in web design towards responsive design. Responsive design is the deliberate design and build of a web site to respond differently to different viewing devices. In other words, the web site will present itself differently depending on whether it is viewed on a mobile phone, tablet or desktop device. The design is done in such a way that the browser itself does the work. This is a great choice in many ways, particularly for smaller web sites. There is only one set of pages to maintain, which makes life simpler, but the styling behind the site actually addresses multiple view ports (the size of the window through which the site is viewed). As a result the underlying code can get a bit complex, even though it’s only in one set of files.
The alternative to responsive design is to detect the device accessing the site and present an alternative set of pages to smaller devices. This obviously will involve more work and more files as you are designing separately for different view ports. At first this may seem to be counter productive, but it all depends on the site content and original design.
Many sites are now quite old and large, and the work involved in redesigning the entire site to a responsive design could be prohibitive. It may well be that you only wish to present a select subset of pages to the mobile user, or even different pages geared towards mobile phone use (such as linking maps and phone numbers).
Many larger sites are also data driven and have a significant amount of back end code that can be easily leveraged to drive new mobile oriented pages with no more work involved than restyling the main pages.
For larger sites, and especially those that have an old design it would seem more productive to produce a subsetted separate site for mobile devices, always presenting the user with the option to use the ‘full fat’ site if they prefer. Responsive design is an elegant design option, particularly when designing from scratch. But depending on the size of the site and the legacy elements, responsive design is not yet the only game in town.